I'd pay 600.00 for a 12' Gregor in good shape with a good 15 hp engine. Heck, I'd pay that for the hull alone if it is in reasonable shape.
The particular examples you're looking at may or may not be good. But I just paid close to $600.00 for a new-to-me 9.9 / 15 Evinrude 2 stroke for my similar but rivited Valco U-12. If I ever do replace this boat, it'll be with another Valco U-12 or the similar Gregor and I'd be willing to spend around a grand to get a nice one. But I really like those two boats and have some sentimental attachment to them from fishing out of Valcos and Gregors all over California when I was a kid. So part of my love for them is down to simple nostalgia, but the other part is that they work well and are well designed and well made little craft.
T-C