Ok so I maxed out the second bike you can unlock, the decimator. To upgrade, push start, select clubhouse, then upgrade bike.
It took about $100,000 to do this and given you will earn around $2,500-$6,000 per mission/race it won’t take long - I was only about half way through the missions.
If you want to get it quickly, use instant missions, escape, and then when it starts pause the game. The timer continues and 30 seconds later, you win - $2,500 a pop, so $100k in about 20-30 mins. Good luck!!