Figure the last run of American made ones (the frames are marked with the DA code) should, ideally, be only slightly more than what you'll buy a new Chinese one for.
The downside is that you'll have to wade through lots that go for way more than they're worth. If you have self control, you'll do fine. I just watched a bunch on my watch list go for much higher than that number, I've been deleting things off as they crossed my threshold to try and keep track of ones I had an active interest in, y'know?
I'd say $30 would leave me smiling considering a new one is ~$25.
The number goes up the older they get, though. The early '30s 1494 cleared $202 when it closed, but was in great shape. The late '39/40 model I wanted went for just over $50.. Quite a difference.
Model matters, too. My new 1498 was $12, and appears to be from the late '50s (six rivet spool). A similar vintage 1495 in awesome shape went for just under $20, too.
The 1492 hasn't changed much at all since the start, though. The biggest thing has been the colour of the plastic and what the spool cover says from the '50s on. Before that, it was the colour of the plastic and a metal spool cover. Meh.
I'd also be willing to go with the Chinese ones if you just want users. The snobs don't want 'em, so they go for less than what you can buy 'em in the stores for. I've gotten a bunch of 1595 spools for ~$8 shipped, a parts reel (read "mistake") and my hackjob (two spools and a frame, shipped for $18?). They all work the same.