So a friend told me about this hack, found a few tutorials on YouTube, @Tano and I had different understandings but I was able to get it working.
1 - Buy a house or a garage, if you have one, cool
2 - Buy a Pegassi Bati 801RR - it has to be this bike
3 - Start an invite only private session
4 - Drive this bike to Los Santos Customs
5 - Sell the bike
6 - Just after you sell the bike, when the money just comes up on the screen, like +1500, press "Start"
7 - Go to Xbox / PSN store, then exit the store
The bike should respawn on the map, and you can do this again and again.
Remember, the more you customize the bike, the most you can sell it for. But the max you can sell it is $50K, but $50K for every 20 seconds will make you very rich very quickly
When you mod your bike, you have to wait 35minutes (1 day in game time) to be able to sell the bike again.
Tried it? Let us know how you go! Apparent it works with the Bugatti as well
I've assigned @Ben to work out how to do this in real life...