Javascript example

HTML

<input type="submit" id="byBtn" value="Change" onclick="change()"/>
 
<input type="text" id="price_start" value="5">
<br>
<br>
<input type="text" id="price_month_1">
<br>
<br>
<input type="text" id="price_month_2">
<br>
<br>
<input type="text" id="price_month_3">
<br>
<br>
<input type="text" id="price_month_4">
<br>
<br>
<input type="text" id="price_month_5">
<br>
<br>
<input type="text" id="price_month_6">

JavaScript

function change(){    
    var price_start = document.getElementById('price_start').value;
    for (i = 1; i <= 14; i++) { 
        constance_val = i*2;
        price_month_+i+ = price_start*constance_val;
        document.getElementById('price_month_'+i+).value = price_month_+i+; 
    }
}