$("#add_pricing").click(function(e){
    e.preventDefault();
    data = {
        "price": "1200",
        "days": ["1","3","4"]
    }
    addData(data);
    console.log(prices)
});

function addData(data)
{
    for (i = 0; i < data.days.length; i++){
        data.day = data.days[i]; //eg. data.day = "1"
        prices.push(data);

    }
}

[1] => {day: 1, price: 1200}
[2] => {day: 3, price: 1200}
[3] => {day: 3, price: 1200}

[1] => {day: 4, price: 1200}
[2] => {day: 4, price: 1200}
[3] => {day: 4, price: 1200}