Money isn't exactly hard to earn either. Set up missions for the third heist pay crewmen $20,000 upon completion. The set up missions, not the big heist itself. The average set up mission length is 10-15 minutes and even winning a 1 minute long race can earn you $3000. Earning money that quickly, the $575,000 bullet proof car doesn't look so far off. I'd know because I got the bullet proof car and the high end apartment without feeling the need to spend any real money on it.
I'm sure there are some loopholes somewhere, but GTA Online continues to prove to me that progression comes from playing rather then playing being about progression. The Warcraft devs have always said that progression is important in an MMO, I believe it. I just don't believe it needs to the point. For now, I'm going to continue my ride in GTA Online.