It depends on your local market more than anything. Do some research by asking around local shops, trying to find a local aquatics club will help in a big way. Different fish sell better in different areas at different times of the year. Around here it's generally slower in the summer, and picks up starting in the fall through the winter.
You won't make a lot of money breeding fish, if you break even consider yourself lucky. I figured out the time it takes me to raise a spawn of angels to sellable size, and what they sell for a while back. I end up making around $5 per hour. This is after all the equipment is paid for. If I was looking for money I'd shut down most of my tanks, and work an extra day each week, I would be ahead dollar wise, but wouldn't have as much fun.
Depending who I'm selling to, angels go for anywhere from 50 cents to $1.50 each, dime to nickel body size. I deal with a broker who buys 200 every other week, he pays $.50, and sells them for $1 each to about a dozen shops he deals with. A couple of local shops pay $1.50 each, but take 25 or 50 every 2 to 4 weeks. If the market is slow nobody is buying anything, and it's easy to get stuck with hundreds of fish nobody will buy, or even take. Brokers pay less, but buy a lot more & come to your house. Dealing with shops takes more time due to having to deliver, as well as sales skills, but they pay better. It all evens out.
Start out slow, and leave extra space in case the market goes dead. I keep a couple of plastic kiddie wading pools in the garage rafters for emergency overstock.