IBM Moder M Keyboards
5 or 6 years ago at computer shows, you could pick up IBM Model keyboads for between $5 and $10 each.
New it's hard to find them for less then $30 (in decent condition)
3.5 inch 1.44MB High Density Floppy Drives.
a 10 year old working floppy is worth the same amount as a brand new one that works.
Cases generally hold their values, however, power supplies don't since the power demands of PCs' keep going up.
Monitors hold their values pretty well currently, compared to say, CPUs or Hard Drives, however, 5 years ago $400 was touching on the very low end of 19inch CRTs. Those monitors now go for about $100 used. (or less)
Memmory prices fluctuate a lot due to the somewhat volitile memmory market. Memmory that was bought in the last 2 or 3 years has maintained its value pretty well, but I remember purchasing a 32MB stick of SDRam for almost $300 back in 96. Nowadays, that RAM chip is worth like, less than 10 bucks.