SO fiddle

Test Fiddle mainly for SO Questions

by Nick Craver

HTML

<link rel="stylesheet" href="http://ajax.googleapis.com/ajax/libs/jqueryui/1.8.4/themes/overcast/jquery-ui.css">
tarif_membre: <input type="text" id="tarif_membre" value="1" />
<br />
total_total: <input type="text" id="total_total" />

JavaScript

$("#tarif_membre").keyup(function() {
    var sum = 12.00 * parseFloat($('#tarif_membre').val());
    total_total= sum.toFixed(2).replace('.',',');
    $('#total_total').val(total_total);
}).keyup();