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 and will have unlimited lives. Keep pressing R and collect as much money as you can. At the end, go back to the shop and check out how much money was collected. Not enough? Just play it again until you get enough.
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 by pressing or holding the R trigger to get even more money. Do not worry about dying because you will have unlimited lives while playing.
Thanks to Revolution reader A Martian!