Try this :

/^\$?(?:\d{1,3},?(?=\d{3}))*[0-9]{1,3}(?:\.\d{2})?$/
[download]

I added (?=\d{3}) for a look ahead, otherwise, it matches things like "2,34,3.56". If you have a comma, you're supposed to have three digits just afterwards. ( I hope I'm being clear enough ).
Moreover, you could add "?:" since you're not capturing any pattern.