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Depends on what you want achieve. If it's weight weenieism then there are better ways of shaving grams.
If it's just what can I spend money on to make it a better bike then for medium money a better fitting saddle and bars could be a nice place to start, assuming the saddle and bars that came with the bike are still on it, and they aren't a perfect fit.
Latex tubes and nicer tyres are always a good start, too.
What you guys recommend for upgrading my caad8 on the cheap? So far I'm thinking lighter stem/seat post, any other recommendations?