While it's always a dream to breed a "special" fish for money, the best way to make money is to breed fish that: #1- your pet shops want to buy, #2- They want to buy them frequently and #3- they need to have them. I've commercially bred many varieties of fish and sometimes, you make more money selling fish that may only bring you say, 25 cents per fish opposed to $1.00 per fish. That's because the shops will sell more cheaper fish than expensive fish. Again, you need to talk to your local shops to see what they want. After you have your routine down and a customer base, you can then start adding "specialty' fish for that extra financial boost.
Hope this helps