money.js

Number.prototype.toMoney = function(decimals, decimal_sep, thousands_sep)
{
   var n = this,
   c = isNaN(decimals) ? 0 : Math.abs(decimals),
   d = decimal_sep || '.',
   t = (typeof thousands_sep === 'undefined') ? ',' : thousands_sep,
   sign = (n < 0) ? '-' : '',
   i = parseInt(n = Math.abs(n).toFixed(c)) + '',
   j = ((j = i.length) > 3) ? j % 3 : 0;
   return sign + (j ? i.substr(0, j) + t : '') + i.substr(j).replace(/(\d{3})(?=\d)/g, "$1" + t) + (c ? d + Math.abs(n - i).toFixed(c).slice(2) : '');
}