Add-on parts are usually worth $0 in resell unless still like new.

Twist throttle = $0
Aftermarket Exhaust = $50
rev limiter = $0
nerfs = $50
bars = $0

paddle tires = $0 unless he can give you the original tires.

Rule is don't mod your bike if you plan to sell it. If you are replacing a stock part, you generally don't get any money. If it is an add-on then you get some allotment (or as the seller you take them off and sell on e-bay).

I would pay up to $3,000 for a "like new" 2001 meaning it has original tires in great shape, no major dings in the wheels. (someone who bought ride and never rode it after a month or two of having it). If it is actually a used 2001 that has lots of riding time then I wouldn't go higher than $2,300. I would go about $300 higher on a 2002.

/Jon