I've gotten them for as little as $5.00 + shipping from Good Will auctions, and as little as $4.00 from a thrift store. My LSMG sells them for $20.00.
Here's one on GW for $5.00 + shipping starting bid:
{
http://www.shopgoodwill.com/viewItem...temID=12333750 }
The one in the green case, rectangle or egg shaped, are for low (straight) shank machines.
Joe