My high-end average seems to hover around the $20 mark, but a find of something good & properly rare would likely spur more money easily. I'd probably be fine dropping $50 on Swordquest Waterworld, for example.
Outside of the big three- 2600, Intellivision, & Colecovision- many of the pre-NES machines seem to exist in a very tumultuous market. Basically, the supply is terribly low- so much even Ebay cannot always help you- but the buying market is equally small. For example, the Fairchild Channel F: it's the first machine with cartridges, so you'd think collectors would be fairly gung ho over it. However, it's also rather hard to come across working machines anymore... it had a short time on the market, and the first revision was hard-wired so you can't just replace a bad cable. As a result, prices for carts (especially rare ones) can bounce wildly. I spent about a year eyeing around for Video Whizball, and saw it go as low as $5 and as high as several hundred on Ebay. With no copies for sale & often no sold units in eBay's search, people were free to set whatever price they wanted. (I ended up paying around $30 for mine on another forum.)
It makes for an interesting era to collect in- good, common games are often cheap & plentiful so building a collection is easy enough- but getting rare games involves a different level of price hunting, & you'll likely spend more time & money on machine maintenance than with other systems.