Typically units go after all the numbers, i.e. 15 miles / hour. Currency can be different as you have indicated. The position of the dollar sign depends on what you are going to do, i.e divide 15 dollars by 100 = $15/100 (or maybe 15 USD / 100); divide 15 by a hundred dollars = 15/$100; one hundredth times 15 dollars = $( (1/100) 15) or maybe (1/100) $15, and so forth.
