No disrespect to Carroll - but he was bought at the peak of the market (playing well for his current side, Jan window meaning lots of strikers were unavailable, thereby raising the price - not to mention window was closing fast I seem to recall)
He was probably a £25m striker on ability (maybe upto £30m) with the other £5-10m being because Liverpool didnt ahve much choice (and still made profit from Torres)
So I would say its fair to say that Carroll now is probably worth only £15m or so (not so prolific, not playing as regularly (is that injury related or purely from form/fitting with the team, and therefore not so important to his current team as he was). In a year's time it could be less, or if form improves it could be more....
for the record if Utd do sell Berba, I think they would be lucky to get £10m from him, and it would probably be less.