Sean,
I finally reached all cars owned and 100% just before the 2015 Le Mans update. I wasted a lot of gold early on and didnt wait for car sales, so I had earned around 18,000 gold at that point.
Before the NASCAR update I was up to around 20,000 gold earned, and had acquired all the LMP cars and the 2015 RSR and was still at 100%. I had around 1,500 gold in the bank (largely due to my daughter and her friends earning me 700+ gold in one weekend when the ads kept reappearing.)
With each of the 4 NASCAR vehicles requiring 800GC in upgrades/PR58 to maintain 100%, the total numbers to reach/maintain 100% just got even more daunting. It's way more than 16-17,000GC for 100% now.
I think I am finally done with RR3. 3,200GC to upgrade 4 identical cars, with fewer free GC from ads, doing endless Le Mans 10 lappers (Past level 450 so need to do seven 10-lappers to level up) just to be able to do endless Richmond/Indy circles? No thanks.