Posted Thu Mar 21, 2013 3:05 PM
I find Super Expander terribly common, so you might just be in a dry spell at the moment and see those high prices.
When it comes to the width of the fingers, I'm not sure if more narrow fingers were more expensive or if Commodore made the cartridges rather wide to look impressive. I know that Atari 400/800 cartridges have more narrow fingers. I never looked at 2600, TI-99 etc to name a few. I figure wide cartridges don't have to protrude as far out from the computer although you see the 24K expansion above that has both width and depth.
As for additional hardware, there were B&W 40/80 column cartridges for the VIC-20 but those have their own video output so the logic screen is redirected to use that hardware. I'm not sure how e.g. sprite hardware could interface with the VIC-I chip inside the computer, probably not at all. Certainly you might be able to interface some different chip, but then you might need to plug in a video cable into the cartridge to play that game.