public class Opcional
{
    public Opcional()
    {
        this.Diarias = new HashSet<Diaria>();
    }
    [Key]
    public int OpcionalId { get; set; }

    [Required]
    public String Descricao { get; set; }

    [Required]
    public decimal Valor { get; set; }

    public virtual ICollection<Diaria> Diarias { get; private set; }
}

<div class="col-md-12">
        <div class="col-md-2">@Html.LabelFor(model => model.Valor)</div>
        <div class="col-md-10">
            @Html.EditorFor(model => model.Valor)
            @Html.ValidationMessageFor(model => model.Valor)
        </div>
    </div>