You can easily get 50% or more of your investment back on aftermarket parts as long as you keep your stock parts. Put all the stock parts back on for the sale of the bike and sell all the aftermarket parts separately. If you buy used aftermarket parts then you should be able to get even more of a return.

Street bikes generally don't hold a candle to the amount of money that is spent on dirt bike aftermarket items. You lose your butt on those if you don't part them separately. Obviously this comment pertains to "normal" bikes, not bikes where NOS stuff is more valuable than any aftermarket replacement.