One other variable to point out is location. If you're in a location where there are other local breeders, the market gets quickly saturated. One batch of eggs can develop into hundreds of babies, and this happens every couple of weeks. You can see how the numbers quickly add up when you add multiple pairs with multiple breeders.
And keep in mind that you're also competing with huge breeding ops that can offer their fish to LFS for pennies. Most LFS carry a few of the common clowns, and typically only a pair or two of each "designer" clown.
As previously mentioned, the more aggressive clowns are even harder to sell. Even those considered somewhat rare such as a McCulloch's clown are a tough sell too -- many people don't find them very attractive.
If you want to breed clowns, and even hope to profit some, my suggestion is to find a niche that works for you. For example, if you can get your hands on a Bali Aquarich Nebula Picasso, you can get creative with your pairing and resulting offspring.
Regardless, your goal should always be to produce the best clowns possible -- extremely high quality (color, form, shape, etc.) with no defects or flaws. This means feeding high quality food (Gresham can give you suggestions, wink-wink) and clean water. Did I mention clean water? ;-) Soon you'll develop a reputation for producing top-notch clowns and quickly build your own brand (a la Rods Onyx or Bali Aquarich P1 Picasso).