whether you paid too much is not an easy question to answer, these forums have people from all areas of all countries, where i am from ps3s are very cheap but ps3s on 3.55 are not easy to get and my 120gig slim 3.55 cost £180 other areas of the country they go from around £150 upto £200. cost seems to be area dictated by area and availability.
as for the repair.. if you put second hand parts in they usually wont last long, or could be faulty when actually purchased, so unless you getting the part very cheap you would be better off buying new.
not very helpful answers, but it wasn;t a very easy question either. (no definitive answer)