I bought mine, advertised as working with 640k RAM (and it was actually working!), for $9.99 on eBay, $20 shipping. So, prices vary wildly. I personally wouldn't pay over $50 including shipping for a good unit.
Yeah, that was part of the draw in my case, that was $75 flat, no shipping cost.
If you ask me, the $10 is all it's "worth." But when I was interested in acquiring mine (and I wanted it to be a 51x0 official, not AMD knockoff or whatever), I could not find any under ~$120. Just now I did an eBay search, I see a 5160 at $130, a 5150 "with VGA and keyboard" for $250, a "*Vintage* IBM 5150 Personal Computer - 8088 CPU & 2 FDD" for a whopping $500. Those are the only 51x0s I saw during this quick search. So while I don't personally feel the machine is WORTH $75 ... it was a good deal compared to the current eBay market.
Obviously you might get lucky here and there (especially those who don't realize that they can sell old things for lots of money), but right now I think there's a stranglehold on diminishing supply of these units, and that's driving the price up. Or I'm just not looking in the right place... I know all the thrift stores back home that used to always have four or five PCs during the "great dumping" seems to have dried up and disappeared as well.
One Good Will store was actually selling these ancient laptops for $1 or $5, in a stack. I bought them all, and through my young curiosity ripped several of them apart just to see how things were done. I remember one was a Tandy laptop and one was the original IBM dual-floppy "convertible" unit. Now see, if I knew then what I know now, those were probably some very nice things I could've gotten a lot more than $1 or $5 for. Need time travel...