What it's worth depends on what year it is (these things were in production for almost 20 years) and what condition it is. If it looks like it wasn't taken care of very well, think twice about buying it. People tend to really neglect quads, they come in low on their list of priorities with maintenance and such. An old quad in rough shape can easily cost you more than a quad only a couple years old in good shape once the repairs on the old one are completed. It's not a stretch at all for a $500 quad to cost you over $1,500 to fix everything that's wrong with it if it has been put out to pasture and not maintained properly.
I'd say a fair price for one in the 99-04 range would be a fair deal for around 1500 assuming it doesn't need any work. Aftermarket parts don't usually add much value but take it into consideration, because if you'd be planning on adding those parts at some point it can save you money, aftermarket parts get expensive real quick.
Don't assume that because it's not old that it doesn't need work, look it over closely. I bought my 2001 in 2005 for $2,800 and by the time the damaged frame was replaced and everything else that was worn out was fixed it was $4,000 quad, and the shop ate the labor charges and 50% of the cost of the frame. That was a life lesson for me.
A few years ago $3,000 was a reasonable price for a 4 or 5 year old warrior. With the increase in popularity of bigger, faster beasts like raptor 660 and 450 racers, and them hitting the used market for the same price it really knocked the prices down on the more basic machines like the warrior and 400ex.
But in general, just make sure that the thing is in as good of shape as it looks, check over the frame for cracked paint and other signs of damage, check the bearings, ball joints, tie rod ends, etc. for tightness, make sure the engine doesn't have any odd sounding thumps or rattles, and use your own judgement. If it seems like it's worth $2,000 to you, then $2,000 is a good price, but you can probably find a good one for less if you look around.
