Hi there,
I may be too late for you, but for conversation's sake, the most I have paid to date is $85. That was for a fiddle base gal with a great skirt, lots of attachments, manual and other goodies included.
Imho, patience pays off. And letting a few go. Recently a machine came up on CL. Price was fair considering the machine. It meant about a 2.5 hour drive one-way. The seller must have had a ton of hits because within less than a couple of hours the price jumped ___ready, wait for it____ $250! Yep. I let that one go. However, while I wasn't looking, a Singer 128 became mine for $21, tax included. The machine is near mint (couple of markings on the solid hand wheel, bent wood case in fair+ - good condition.
$350 for a Singer 66 in my opinion is way too high.
best,
d