Well, if we stick with ESB characters on Logo'd cards....
The most sort after are the 30 backs, but they aren't necessarily the rarest.
They used to be quite cheap, but a sudden rush of interest in them from about 2000/01 has caused the price to go through the roof.
A recent no eBay auction saw many hit well over Ã'£500 and some reach Ã'£800+
The hardest and dearest would probably be......
Fett
Yoda
White Bespin Guard
Han Bespin
FX7
Hoth Stormtrooper
IG 88
Bossk.
You could be looking at thousands now for a Fett.
41 and 45a tend to be cheaper. Often by quite a bit. Though some are very tough/expensive to get.
Luke Bespin can be tough on 41 whereas he's fairly easy on a 30 back(though still pricey). Fetts don't come up too often. Most of the others that can be tricky are the same as the 30 backs, though Yoda is realtively common on 41.
Things like Imperial commander, Leia Hoth, Han Hoth, Lobot and AT AT driver pop up quite regularly.
45a's have some of the most common figures. BBG and AT AT commander. Though try finding a nice crystal clear BBG!!! Most of the debut 45a's are easy enough. Luke Hoth can be tricky as can CCP
But Dengar is a nightmare and C3P0 removeable limbs is even worse!!!
Hope that explains a few things.
:?