One other observation is that the quality and extent of the modifications are usually proportional to the cost of the car.
As a casual observer, I'd have thought the reverse; it seems to me mods are prevalent on certain low base price cars, Civics, Sonics, and the like, where you have relatively little investment to ruin; do people really do a lot of modding of $50K+ cars, where you're talking, theoretically, about an optimally designed and well constructed machine?
I'd also, though, think that the degree of modding would have something to do with the disposable income of the modder. I wouldn't have dared do serious mods to my only car, but when I was able to afford a second ride, I suddenly felt free to tinker around with it, because having it sit in a shop somewhere for weeks at a time (as has already happened multiple times to me in less than a year's ownership) isn't crippling.
And, perhaps, marital status; a spouse might limit mods by mentioning that instead of a Turbo Upgrade it might be wiser to save for little Jimmie's college education... or want to waste it on something frivolous like paying down the mortgage.