I wouldn't pay any extra for mods. I did buy a car once that had BBS rims, the owner wanted an extra $200 for the rims, but I told him to put the stock rims on, so rather than go through all that, he just threw them in and gave me the stock rims.
In general I stay away from cars that have mods, it just raises the purchase price and I think I hate most mods out there. I hate bigger rims, the lowered look, body kits, the murdered look, etc., I think it pretty much all looks worse. Factory is equivalent to off white paint on a house, most people might not love it, but different colors like mods means it's either a love or hate relationship. I just prefer bone stock and the lower price that goes with it. And it's all about supply and demand when it comes to sales, make too many people hate it and the number of people who love it are very small and don't create the demand for the higher price. Even those that love it know they can get the guy down lower.
Oh and while go fast pieces are nice, doesn't that just mean the car was probably flogged to within an inch of it's life and maybe won't last much longer? Everyone is sorta looking for the car that granny drove to church on Sunday not the one that was driven hard. Mods just scream lowball me, and it's funny when the ads say no low ballers.