To get simple money just go to arcade mode and when you start the race
don't try to go fast and win. Start out by doing a bit of combat (circle
or square) then stop once you have been passed. Start doing a bunch of
tricks. For example manuals, stoppies, superman's, back flips ect. If
you can go off the same jump twice. Then when you finish the race you
should get lots of money for tricks. With this money you can get things
for your bike in single player.