string myConnection = "datasource=localhost; port=3306; username=root;password=root";
                if (comboBox4.SelectedItem.ToString() == "Salary")
                {
                    string insert = "insert into budget.salary (name, suma) values (@name, @price);";

                    using (var conDataBase = new MySqlConnection(myConnection))
                    using (var cmdDataBase = new MySqlCommand(insert, conDataBase))
                    {
                        cmdDataBase.Parameters.AddWithValue("@name", name);
                        cmdDataBase.Parameters.AddWithValue("@price", suma);
                        conDataBase.Open();
                        cmdDataBase.ExecuteNonQuery();

                        MessageBox.Show("Saved");
                    }
                }
                if (comboBox4.SelectedItem.ToString() == "Other")
                {
                    string insert = "insert into budget.other (name, suma) values (@name, @price);";

                    using (var conDataBase = new MySqlConnection(myConnection))
                    using (var cmdDataBase = new MySqlCommand(insert, conDataBase))
                    {
                        cmdDataBase.Parameters.AddWithValue("@name", name);
                        cmdDataBase.Parameters.AddWithValue("@price", suma);
                        conDataBase.Open();
                        cmdDataBase.ExecuteNonQuery();
                        MessageBox.Show("Saved");

                    }
                }

public void InsertInto(string table) {
  string myConnection = "datasource=localhost; port=3306; username=root;password=root";
  string insert = "insert into budget." + table + " (name, suma) values (@name, @price);";

  using (var conDataBase = new MySqlConnection(myConnection))
  using (var cmdDataBase = new MySqlCommand(iterpti, conDataBase)) {
    cmdDataBase.Parameters.AddWithValue("@name", name);
    cmdDataBase.Parameters.AddWithValue("@price", suma);
    conDataBase.Open();
    cmdDataBase.ExecuteNonQuery();
    MessageBox.Show("Saved");
  }
}

if (comboBox4.SelectedItem.ToString() == "Salary") {
    InsertInto("salary");
}
if (comboBox4.SelectedItem.ToString() == "Other") {
    InsertInto("other");
}

string tableName = null; // tableName is the only difference between the two, right?
if (comboBox4.SelectedItem.ToString() == "Salary") {
    tableName = "salary";
} else if (comboBox4.SelectedItem.ToString() == "Other") {
    tableName = "other";
}

if (tableName != null) {
    string iterpti = $"insert into budget.{tableName} (name, suma) values (@name, @price);";

    using (var conDataBase = new MySqlConnection(myConnection))
    using (var cmdDataBase = new MySqlCommand(iterpti, conDataBase))
    {
        cmdDataBase.Parameters.AddWithValue("@name", name);
        cmdDataBase.Parameters.AddWithValue("@price", suma);
        conDataBase.Open();
        cmdDataBase.ExecuteNonQuery();

        MessageBox.Show("Saved");
    }

}

private void InsertIntoTable(string tableName) {
    string iterpti = $"insert into budget.{tableName} (name, suma) values (@name, @price);";

    using (var conDataBase = new MySqlConnection(myConnection))
    using (var cmdDataBase = new MySqlCommand(iterpti, conDataBase))
    {
        cmdDataBase.Parameters.AddWithValue("@name", name);
        cmdDataBase.Parameters.AddWithValue("@price", suma);
        conDataBase.Open();
        cmdDataBase.ExecuteNonQuery();

        MessageBox.Show("Saved");
    }
}

string iterpti = "insert into budget." + tableName + " (name, suma) values (@name, @price);";

private void DoInsertion(string tableName)
{
  //logic to insert data
}

if (comboBox4.SelectedItem.ToString() == "Salary")
                {
                    DoInsertion("Salary");
                }

string myConnection = "datasource=localhost; port=3306; username=root;password=root";
string value = comboBox4.SelectedItem.ToString();
string insert = "";

switch (value)
{
    case "Salary":
        insert = "insert into budget.salary (name, suma) values (@name, @price);";
        break;

    case "Other":
        insert = "insert into budget.other (name, suma) values (@name, @price);";
        break;

    # This case is optional in this situation.
    # You may not have to use this.
    default:
        break;
}

using (var conDataBase = new MySqlConnection(myConnection))
using (var cmdDataBase = new MySqlCommand(insert, conDataBase))
{
    cmdDataBase.Parameters.AddWithValue("@name", name);
    cmdDataBase.Parameters.AddWithValue("@price", suma);
    conDataBase.Open();
    cmdDataBase.ExecuteNonQuery();

    MessageBox.Show("Saved");
}

string myConnection = "datasource=localhost; port=3306; username=root;password=root";
string value = comboBox4.SelectedItem.ToString().ToLower();
string insert = "";

if (value == "salary")
{
        insert = "insert into budget.salary (name, suma) values (@name, @price);";
}

else if (value == "other")
        insert = "insert into budget.other (name, suma) values (@name, @price);";
}

using (var conDataBase = new MySqlConnection(myConnection))
using (var cmdDataBase = new MySqlCommand(insert, conDataBase))
{
    cmdDataBase.Parameters.AddWithValue("@name", name);
    cmdDataBase.Parameters.AddWithValue("@price", suma);
    conDataBase.Open();
    cmdDataBase.ExecuteNonQuery();

    MessageBox.Show("Saved");
}

string myConnection = "datasource=localhost; port=3306; username=root;password=root";
string budgetField = comboBox4.SelectedItem.ToString().ToLower(); // This will be either "salary" or "other"

// This is where the magic happens:  The {bracketed} variable gets repaced with either "salary" or "other".
string insert = $"insert into budget.{budgetField} (name, suma) values (@name, @price);";

using (var conDataBase = new MySqlConnection(myConnection))
using (var cmdDataBase = new MySqlCommand(iterpti, conDataBase))
{
    cmdDataBase.Parameters.AddWithValue("@name", name);
    cmdDataBase.Parameters.AddWithValue("@price", suma);
    conDataBase.Open();
    cmdDataBase.ExecuteNonQuery();
    MessageBox.Show("Saved");
}

public class Example1
{
    private const string ConnectionString = "datasource=localhost; port=3306; username=root;password=root";

    public void ExecuteOperation(string valueCombobox, string name, double price)
    {
        var sql = ConstructSql(valueCombobox);
        var value = InsertOperation(ConnectionString, sql, name, price);

        //Do other things overhere like display the success message since you have the value variable.
    }

    private string ConstructSql(string valueCombobox)
    {
        return string.Format("insert into budget.{0} (name, suma) values (@name, @price)", valueCombobox);
    }
    private int InsertOperation(string connectionstring, string sql, string name, double price)
    {
        using (var connection = new MySqlConnection(connectionstring))
        using (var command = new MySqlCommand(sql, connection))
        {
            command.Parameters.AddWithValue("@name", name);
            command.Parameters.AddWithValue("@price", price);
            connection.Open();

            return command.ExecuteNonQuery();
        }
    }
}

private void Form1_Load(object sender, EventArgs e)
 {
SqlConnection connection = new SqlConnection();
SqlCommand command = new SqlCommand();
SqlDataReader reader;

connection.ConnectionString = @"Data Source=.SQLEXPRESS;Initial Catalog=University;"
                             + "Integrated Security=SSPI";
command.Connection = connection;
command.CommandText = "SELECT FirstName, LastName, Age FROM Students";

connection.Open();
reader = command.ExecuteReader();

while (reader.Read())
{
    string firstName = reader["FirstName"].ToString();
    string lastName = reader["LastName"].ToString();
    int age = Convert.ToInt32(reader["Age"]);

    studentsListBox.Items.Add(String.Format("{0} {1}, {2}",firstName,lastName,age));
}

reader.Close();
connection.Close();