Kartboy front bushing and rear stay are my initial must do mod for under $100.
Aside from that other things I've done for under $100 to drastically improve the car:
Polk DB6501 components for the front sound stage. I found them for $99 shipped.
TSX-R lenses for the STi projectors. Make for a ridiculously improved output. Must buy for anyone with an 06+ STi or with STi Projectors. $50