A slight modification to the code of Taiki.
Because if user enter 1000000 it will produce 1,000,000. but if the user use Backspace key to remove "0" it will not work.
function addPriceFormat()
{
var numb='';
nStr = document.getElementById('txt').value;
my = nStr.split(',');
var Len = my.length;
for (var i=0; i<Len;i++){numb = numb+my[i];}
x = numb.split('.');
x1 = x[0];
x2 = x.length > 1 ? '.' + x[1] : '';
var rgx = /(\d+)(\d{3})/;
while (rgx.test(x1))
{
x1 = x1.replace(rgx, '$1' + ',' + '$2');
}
formated = x1 + x2;
document.getElementById('txt').value = formated;
}