You have to be selective, otherwise you'll go broke, lol.
Some diecast increase in value, but the bubble can bust, most times than not you'll be lucky if you can recover what you paid because so many were produced.
Higher end, limited production NASCAR diecast were getting stupid priced when the NASCAR diecast market was peaking in the mid 2000's. These days you can pick up the same high end cars for less than half the price they were going for new, never mind the flippers that got them and instantly jacked up the prices when they were hot.
Incidentally, I found a Greenlight yellow Track Attack GT350 to add to my collection of four I got last week. lol