I wouldn't consider a 750 over a 620 - for the power difference the price difference is not justifiable (of same year, cond., etc...) And as far as I know the 750 and 620 are the same physical size - correct? I have a 2003 620 and love it - I wouldn't mind having a 900, but then again I am a fairly laid back rider and am not concerned with the hp's most of the time so I wouldn't really benefit from that. So that just depends on what you are capable of handling, or are interested in.
As for the price on the 2001 Monster - try Kelly Blue Book - they seem to be close on most vehicles I have looked up. Also check cycletrader, the classifieds here, places like that to see what the bikes are actually going for.
Good luck - I say go for the 620, but I obviously am biased ;D - what can I say, I LOVE my Monster.