what is the best $100 (or under) upgrade you've made ?
with that please include the worst upgrade you've made ($100 or under)
whatever kind of bike you ride road ,tandem ,recumbent, MTB -any.
my best would be parallel-push xt front and arch rival rear v-brakes for my mtb(e-bay kept it under $100).
my worst would be aero bars without armrests for my road bike.