There were a few different companies that made them pre-revoltech, including Kaiyodo (who makes the current revoltech options). Sega made some, kaiyodo made some, Bandai made two different flavors of model kits (as well as a perfect grade eva-01), and there were some smaller manufacturers that had offerings as well (Yamato? maybe?).

As far as the old Kaiyodo/Xebec monoshaft ones go, which were the ones that had widest distribution and are the ones you probably have, I really don't think you'd be able to get all that much for them. Sometimes MOSC ones show up on ebay and they go for $15-20 sealed for the regular (non deluxe) figures. That's IF someone actually buys them of course. There's probably not a lot of demand for them as they're pretty inferior to just about every other modern evangelion product out there, unfortunately.