Nepenthes Hamata, villosa, macrophlla could eat your money.
macropyhlla is a bit spendy, but villosa is incredibly cheap now...and hamata is getting rather cheap in comparison to what it was a few years ago....
Now, definitely a mature edwardsiana would hold a hefty price tag, as a near mature one was just sold on another forum for a few grand IIRC...
any other neps that are "rare" would roll in bucks, mainly things you dont see in cultivation, TC or in large quantities....
I.E edwardsiana, klossii, pitopangii, attenboroughii, hurrelliana, the TRUE thorelii, etc....
sure, cephs are "valuable" but not as much so as they used to be...the only ceph that would fetch insane cash now, is Eden Black, or specimens that fill a very LARGE pot...
but as others have stated, dont collect plants because of their value, collect them because you like them...