there is a treadle yahoo group that you might want to join...
also this one:
http://groups.yahoo.com/group/vintagesingers/
yes it is a 'treasure' - but only to you - because it was your grandmothers and it is in good condition... as far as monetary value, they don't sell for much because so many were made...
that said, I paid $300 for one (an exorbitant price by the way) but the cabinet had been totally refurbished, and the machine had been completely cleaned etc - in other words the whole thing is in fantastic condition -
would I pay that for one in a ratty cabinet that was rusty? no, or one that was 'ok' but needed work? - no
join that vintage singer group to find out how to clean the machine safely, there's a lady on there that sells the belts, you can download the manual (singer 66) from the singer site, etc