Look up the Toyota price for the Cat and show the seller that number, if you could get at least that much off you might be in the ballpark. Remember that colorado law requires the vehicle to pass emissions when sold and this one will not so you should be in a good negotiating point. Offer ~$3k and start walking maybe?
Also recall that the frame warranty is up on that one so that is something you want to look at very carefully before you buy and may be another negotiating point.
I have a 2001 with a 2.7 and 115k-mi, nice suspension, and a superbad sound system but I wouldn't expect more than about $5.5k for it (but I'm not selling