If you still have the original sticker, use an on-line appraisal (such as
New Cars, Used Cars, Car Reviews and Pricing - Edmunds.com) to estimate today's private party value of the base F150 (XLT, Lariat, whatever) and determine the % depreciation. Apply that % depreciation to the Roush sticker price to come up with a value.
Let's say a 2001 2WD / 4 Dr Lariat with the options/ mileage you have is worth $7,500 today and the original sticker price of the Lariat was $25,000, the depreciation would be 70%. So as a ballpark, the truck is worth 30% of the original Roush sticker.
My guess is that approach will be the high side. The low side would be the current value of the base F150 with no added value for the Roush package.