I picked up my Saunders 4-blade 14"x17" from eBay for under $50. The only problem was that some of the black had worn off of the top of the blades. A bit of black paint and some wet-sanding was all it took to get it back into perfect shape. So far it's worked the way it's supposed to, holding the paper flat and keeping the blades square. No complaints about the blades staying in position.
If you're serious about getting an easel cheap, look for the ones with cosmetic problems and ask the seller detailed questions about the mechanics. (I've used this same tactic with lenses...worn barrels with perfect glass go for a lot less than perfect barrels with perfect glass.)