This is very easy method to earn lots of money.

1) Visit Costalots general store.
2) Purchase the red chili seed.
3) Place two lots of four very close together and another two close together.
4) Buy the red fertilizer.
5) Put the red fertilizer over the three clusters of chili seeds.
6) Water the plants until the chilies lay on the floor. (They will be very large than an average chili.)
7) Go to Costalots store again.
8) Select "Sell".
9) Sell all of the chilies to her.
10) She will pay you $400 per chili, and since you grew the maximum of ten you will earn $4000 every time.
11) This glitch can be repeated as many times as you want until you have as much money as you like.

Note: If you plant ten and then pay for them you can plant more. Doing this you can earn much more than $4000 every time. Possible up to $20,000 per harvest.

If you have any problems here is a Youtube video explaining: