When buying a SP1, you really want to make sure it is an original SP1. Some have been presented as SP1s when, in fact, they were not. They would have a SP1 lower and a different upper on them. For an original Colt Sporter SP1 in half way decent condition, $800 is not a bad price. The price also depends on whether it is a carbine or a rifle. I have a 1977 Colt Sporter SP1 in pristine condition that I paid $1K for about six years ago. It does not look like it has ever been shot. I was fortunate to get it at the time. It would now be worth a few hundred dollars more because I have never shot it.
If you can get some detailed pictures of the one you are looking at, it would help in determining its value. The serial number can be looked up to find out when it was manufactured.