Regular expression - only digits, dots,commas and first character should be $

I would like to have a regex for the following expression

$123,456,789$1.23$123

what i have done so far is if use enter 123456789 i will convert this to dollar format as follows $123,456,789.00

But i would like to valid the above values if he enters in a text box. I would like to allow only . with the numeric values. The $ symbol will come after my conversion but at the time of leaving i would like to validate the text box as per the requirement any idea please