[Benjo]

I don't think it is possible to actually edit the menus and make the modifications free. However, I am sure that a clever piece of behind-the-scenes math could temporarily increase the players' cash when requested, and then reset to it's original value once the vehicle modification has been applied. It would be barely noticable to the player.

You should probably take note that while most modifications in these menus call OnVehicleMod, things like colors and paintjobs will not call their respective callbacks until the player exits the garage. I can't think of any way of making the colors and paintjobs free, other than using seperate /color and /paintjob commands.

Hope this helps.