If you are interested in a lifted truck, then the cost of the lift kit and installation would be added to the resale price. The value of the lift depends on if it was installed by your cousin or professionally.
Did your cousin install the lift kit himself? If so, it could have run him anywhere between $400 to $4,000. With professional installation, it would have cost him between $800 to $8,000. I suggest asking for a receipt of the work or the brand name of the kit, so you get a better idea if the sale price of the truck is fair.