The higher spec ones are newer hence more expensive. Intel Core 2 Duo is as far as I know meant to be better than Intel Pentium dual core but has been out long enough for the price to drop.
So if it was me, personally, I would be looking at either an AMD dual core or a Intel Core 2 Duo.
I might be wrong about the Intel but I definatly wouldn't be being quad core they are too new and too expensive and don't give you any real advantage.
Pick a couple and compare them here click here hopefully the ones you choose will be listed to give you some idea of performance versus price.