I will tell you how to get $12,500 in 6 minutes.

To get the money you you will need to go to level 5-2.

At the start and after you beat the Lickers, they drop Lion Hearts that give you $2,500 each.

Aell all 5 and you will get $12,500. After you beat the Lickers there will be a door that you have to both press B on.

After you press it, you will go to a metal factory. Then after it saves quit, sell the Lion Hearts and just keep killing the Lickers over and over.