Most treadles I figure $50 - $250, depending on condition. If it's something worth a lot more, then I normally pass it by unless it's something I know quite a bit about - and know what they go for on eBay.
i was looking at craigslist in the Kansas City area, there are some really nice machines listed right now, I'd have no trouble filling the van . . . but I'd have a lot of trouble figuring out where to put everything once I got home!