As a hobby breeder there is only one way to make money from breeding. You need to work with the rarer, pricier fish. I manage to pay my entire hobby costs by doing this. This means you need to be prepared to spend anywhere from about $1,500 to $3,500 just for your breeding stock. You can save money here by buying very young fish, but then you won't be certain of the sexes and you will spend a year or two growing them to spawning size.
Next, choose fish that stay small. Big fish need lots of space and that costs.
I suggest you take a look at AquaBid.com. Look at the listings under Catfish which are Corydoradinae, Plecostomus and Others. There are 4 pages of plecos with about 190 listings and 11 have bids. Corys have about 78 listings and two bids. Others has about 98 listings and 1 bid.
If you want to learn about spawning catfish, then Jools gave good advice re plecos about starting with bristlenose. There are inexpensive ones you can learn with and after that you can decide what you really want to do. You can buy stock for 5 or 10 a fish, they are fairly easy to keep and not difficult to spawn. Learn without risking a lot of money and then once you know what is involved, you can make an informed decision about how to proceed from there.
Bear in mind that the price of fish rise as they become more rare, harder to collect in the wild, take longer to reach spawning age and which produce smaller broods. They also rise as fish move from being babies to juvies, to adults from being unsexed to sexed and finally the most expensive are proven breeders.
Finally, there are two parts here- breeding and selling. The second one is as important as the first.
Two last observations. First, before I was willing to take the risk of moving into the more expensive fish, I had been in the hobby for 5+ years and had a number of fish spawn. I broke into my IRA to pay for them. I got lucky and it worked out well. Second, I never could have done what I have with plecos if it were not for some very excellent help from a number of people who had one thing in common, they are all members here. So if you proceed and you use this site to help you succeed, please consider donating some part of your sales to the site.