Easy money:
Buy Score Attack 3 in a shop, then go to arrange mode A and select Score Attack 3. Choose the Mosquito 02 (the blue jet) because it is much faster then Mosquito 01. You will die in the game will have unlimited lives. No matter what happens, keep pressing R and collect as much money as possible. At the end, return to the shop and check how much money was collected. If not enough, play it again until you get enough.

Hint:
Try to buy the Speed Option and Score Attack 4 in the shop. Then make it fast for your jet, by going to Special Option. Go to Score Attack and choose "Arrange" mode. Then select Mosquito 02 (the blue jet) so you can move fast. Kill everything in sight pressing or holding R trigger to get even more money. Do not worry about dying because you will have unlimited lives while playing.