I've dumped many thousands in mods on various cars over the year, and at the end of the day didn't really make my rides that much more enjoyable, since I ended selling them anyway. Sure they were much faster, and louder, and different looking, etc, etc. But in hindsight it was not worth it.
So I guess I'm in the minority to say that I'm done with this game, and prefer spending the cash on a nice vacation with my family, or save the money to buy a better ride next time around, rather than turning my cars into something they were not designed to be (I don't have the urge right now, still very happy with the M3.)
Not trying to judge anyone, I know the feeling. I guess I'm a recovering modder