Joe, there's no standard price on what a 206 ought to cost. I paid roughly fifty each for the four I own, all sourced from Craigslist in the NYC/Tri-State Area, and they all came in rough shape.
They are in fact Singer's first domestic zigzags. The four that I've got are aluminum; I'd heard they also made them in cast iron, but haven't been able to confirm that.
I'd be happy to sell one or more of my 206s, or at least quote you a price, but I must admit I'm probably going to get to refurbishing them later rather than sooner; I've got a glut of 201-2s and a couple 221s I'd like to see off first.
If you keep your eyes peeled on your local Craigslist, you're bound to run across a 206 sooner or later. None of the ones I picked up were advertised as 206s, it was always a vague "Singer sewing machine" description in the ad, and I'd recognize the 206 in the photo and race over to pick it up.