I've seen 8MHz ZipChips sell openly for around $80. A few years ago, a seller was purging several of them and after getting top dollar for most of them, demand slowed down and the later auctions sold for considerably less.
Here's a tip you may already know - look closely at every machine auction, especially when the lid is popped. Check the CPU socket and for any cards that are installed for something 'bonus' the seller isn't aware of. I've nabbed more Transwarp cards on the cheap from broken, parts only auctions than from any other source. Recent example, I bid on 2 broken, 'won't boot' machines from a recycler on eBay - got them for $65. For that, I picked up 2 Transwarp cards, 2 1MB RAMWorks w/512K add-on cards, 1 PC Transporter, 1 SerialPro and 1 RAMFactor. One of the Transwarp cards needed a new CPU to make it work -- all the other cards worked right away. The IIe machines themselves are too beat up for my collection and are heading to this year's KansasFest for giveaway.