I bought mine on eBay about 15 years ago for $25- the shipping/handling/packing fee was $35! Then I had it tuned up and cleaned for another $20. And I found a used sewing cabinet from the same era at an antique shop for another $45- the 15-91 doesn't really 'fit' but it's very usable if I don't try to fold the sm down into the cabinet. (I *had* to have the 15-91 since it was sooooo pretty and we were both born the same year!) I was- and am- most happy with it and the costs. Spend what your heart will be pleased with investing in the little darling. What anybody else thinks isn't really all that important.
Check eBay and Craig's list and a few places for a guideline, but your heart rules when it comes to a sm that makes your heart flutter and you smile from ear to ear! ;-)
Leslie & The Furbabies in MO.