I have a very good idea on how to make easy money in GTA online.
You get a group of 6-8 people who all know each other (ideally in the same crew). 1 has to be very high leveled. 1 has to be as low leveled as possible. All get on a mission together, either a race or a deathmatch.
Everyone max bets on the low level guy. Let him win. Odds will typically be better than 20 to 1 payouts in this scenario. So, if you bet $2500, you're getting 50K plus each time.
Find a quick mission and your could make a million in an hour or so.
I still haven't figured out exactly how the betting system works, but I'm pretty sure this won't work. It seems to change the odds right before the match starts.. for example I've seen my odds at 4/1 to win a race and placed $2500 on it, only to see it jump to 2/5 at the last moment. Instead of winning 10k I only received 1k. I also think bets on other players must be placed, or you'll get the "not enough bets placed" message.You can still make money with a group of friends by betting on a race/deathmatch and letting a specific person win. I just don't think it's quite as lucrative due to the weird betting rules.
I've found that doing the survival mission at Boneyard and staying in that spot behind the trailer is a very easy way for a group of four to get 20k each fairly quickly. Plus tons of XP points.