hogans alley is about one of the most common games out there (at least in my experience). As far as what i would mostly be concerned with there would be if the gun is in good shape and is accurate. The guns are a pain to find and may run you about 100$ to replace, although you can hack up an old nes zapper and get it to work.
As far as swapping out for other games, most vs. Games are 6 chips and a ppu (graphics processor). The 6 chips can easily be reproduced and burned by many people, but the ppu is the important piece.
I wouldn't swap them too often without just buying another nintendo vs. Motherboard as each time you swap you may damage the eproms (break a leg, etc) and the motherboards hold two games and are only about 40$
now as far as price comparisons. I sold my hogans alley to a member here for about ... I think 350$ and the cabinet was in good shape, monitor looked good, gun was accurate, and i threw in a control panel populated for other vs games. I was on the low side of things however.
For a lot of information visit
www.johnsarcade.com (blkdog7's site) it is a wealth of nintendo vs. Information.