The values for old Macs aren't really all that clear. It used to be we could find the values in a computer price index, but old Macs have been left off that list for a long time. They also aren't considered collectibles by most. You won't find guidebooks or dealers for them like you would cards, stamps, or baseball cards.
eBay is often an inaccurate way of measuring value. Many of the 68Ks on there are overpriced--I've seen Classics and Pluses for several hundred dollars despite being in poor condition. Also, the ones that receive bids often seem to get them for no particular reason. Case in point--when I bid on my PB190 two weeks ago, I was the only bidder on the machine. It cost $30 and was in great shape. There was also a lesser 190 in terms of condition (it was very obvious from the pictures) going for $10, but that one had a bid on it long before the one I bought did.
Your best bet is to search the trading post forum here to see what has sold lately.
eBay is often an inaccurate way of measuring value. Many of the 68Ks on there are overpriced--I've seen Classics and Pluses for several hundred dollars despite being in poor condition. Also, the ones that receive bids often seem to get them for no particular reason. Case in point--when I bid on my PB190 two weeks ago, I was the only bidder on the machine. It cost $30 and was in great shape. There was also a lesser 190 in terms of condition (it was very obvious from the pictures) going for $10, but that one had a bid on it long before the one I bought did.
Your best bet is to search the trading post forum here to see what has sold lately.