Much of the above is from an American perspective. Prices in the UK second hand market and on Ebay.co.uk are higher here.
Recently, for a project I have of using cheap Nikon AF bodies, I have been monitoring Ebay and shop prices. While second hand camera shops prices seem to have stagnated over the last year and some tendency to lower prices, Ebay prices recently have gone up a bit.
There used to be a stream of cheap Nikon F50, 55, 60, 65, 75 and the older F401, but now prices have gone up to 20-30 for F55, as an example. Even the older F401 prices seem to have gone up and can fetch 30+.

Some have mentioned the F75. It is a very good camera. Bought one almost 2 years ago, but it came with a chip on the outer ring of the mode dial. Just a cosmetic flaw, fully functional. Paid 25 for the body. Added the MB-18 grip for 37. My other F75 came this year at about 35 and in excellent condition.

Just a few titbits: although they were the bottom of the Nikon range at some point, some had nice features such as a MLU with the Time function. It is the case of the Nikon F50. Some could show you the automated DX ISO setting like the F60 and the F65 was the first Nikon to come with a wireless remote control.

For the asking prices now, the Nikons F70 and F80 are probably the best deals. The F80 would be a better deal if it had exposure control in 1/3, like the F70 has, and the metering knob wasn't so fiddly. The F90X, being a semi-pro camera still can be had in the UK for very reasonable prices, although they have gone up in value recently.