'$ ' + formatFloat($Expense/MyTotal, '###,###,###,##0.00')
The 3 zero's at the end will force the number to always show these decimals, regardless of whether they are there or not.
10000 will come out as $ 10,000.00
edit: Edited the answer to include Lex's comment. Thanks Lex!
By the way, the above doesn't work when a value is $ 0.00, since it will display $ .00, so I used
if $Expense/MyTotal = 0 then '$ 0.00' else '$ ' + formatFloat($Expense/MyTotal, '###,###,###,###.00')
and that fixed that problem right there.