Not everyone that can afford a 60-70K car could just make the jump to a 100K M5. There are less fortunate people out there
That plus other reason such as xdrive, gas mileage, and ones needs for a day to day car. For some 300 or 400hp is more than enough hp but want to modify their car for strictly aesthetic reasons. Adding a trunk lip spoiler and quad exhaust doesn't make someone trying to be a M5. Lots of people have been doing these types of mods long before the M5 even came out here in the states.
I do agree that i've seen some 550 owners spend well over 10K on just Dinan mods on a car that already stickered for 80+. That doesn't make sense to me unless you need xdrive because you are already up in M5 pricing territory.
Alan