Boat value is really hard to say without seeing the boat. Also, local supply and demand will have a lot of effect on price too.
I picked up my first used 17 for $1k. It was a 1989 boat and included a set of beach wheels but had no trailer and the sail was garbage. It also had some other "issues". Still, I think it was a reasonable deal since I had a lot of parts available to me to get it up and running and I had the know-how to fix it myself.
The cost of a replacement sail is going to be a big factor. You have to figure that a new sail from Hobie costs over $1000. Finding a decent used 17 sail is probably going to be tough. The problem is that the older ones would delaminate making them worthless. The new ones are good, but I don't see a lot of turn-over on them. Most racers I know tend to keep their sail for a long time.
Depending on condition and with a trailer, but no sail, I would think the range would be from $1200 for a complete but fairly used boat up to $3500 for a fairly new boat in very good condition, maybe a little more if it's immaculate. You really have to look at the total price someone is going to pay to get the boat on the water.