If it were me, maybe $800 tops, assuming it has a title. You have to look at how much the repairs are going to cost, and add that on top of the asking price. What is your time worth if you fix it yourself? Maybe its a quick easy fix and you come out ahead, maybe not.
The other thing I would look at is, how much of the bike are you going to keep as is? Are you leaving the tank? I would assume most of accessories are coming off, as well as the seat. Can you make any money back selling them on e-bay or anything? And, how much are the pieces going to cost to make it the way you want? If you have to drop a few hundred dollars, (and I think we all know that is a a conservative estimate), is it worth $1500 or more to you?
The point is, sometimes you are better off with a bike in a little worse shape, that is still be mechanically sound, you can pick up cheaper. You are still going to have to put money into it, can you get away with starting with a cheaper platform? That bike looks very nice, and (it looks like) someone has put quite a bit of care and maintenance into the bike. Maybe it is a little more put together than you need to start with?
Can you tell us a little more about your vision?
Later,
Ryan