Just in case this helps someone else searching for this information, here's the expression I ended up using for my HUD:
set text to "= " & left(str(money*100), len(str(money*100))-2) & "." & right(str(money*100), 2)
There is one small weakness with this method: if the number is zero then there will be no decimals. But that one exception can be easily taken care of with an if-statement.
EDIT: Actually there is the problem that if the original value is less than one (for example a decimal like 0.50) then we only get a number with two digits and the above expression stops working. I need to research this a bit more.