ive made a video on how to get as much money as you want in gta v.
just need the most expensive bike $15,000 then put lots of money into it and keep reselling it.
You need a garage, and if like me had nothing youll need about 80,000 with some bike races won to help sell the bike for more
since thats how you ass more upgrades. This still works as of now, take a look.