12 bolt question

[Quadracer392]

I have the weaker 8.2 10 bolt rearend in my car and I'm wanting to put a set of gears in it with a posi unit but after paying for the posi and gears I'm probably going to be around $600-$700 just in those parts. I found a 12 bolt on craigslist about an hour from where I live, which will be a lot stronger than my 10 bolt. It has 4.11 gears and posi already in it. The guy hasn't said what he wants for it and to make an offer. What would be a good starting offer price? I was thinking about starting my offer at 5-$700, would that be reasonable or to high? I've seen prices on CL from other areas anywhere from $200 clear up to $1500. Here's the only pictures I have of it.