Jane Fonda or aerobics instructor of the 80's.
It's easy and cheap. All you need are tights, a leotard, sneakers, leg warmers, head-band, wrist-bands and a very thin belt. If you are making the costume for a baby you can use a onesie instead of the leotard.
What's important is to get the colors right: they must all be very 80's (fucsias, fluorescent green, etc). If things can be striped... all the better.