I raise chicks to the point of not needing heat, about 4 to 6 weeks, and then sell the pullets. Cockerels are pretty obvious from my flock at that point. And lots of first timers are nervous about raising day olds. They prefer to buy chicks they can be sure of, in terms of gender, and not have to worry about killing their new babies with too much or not enough heat. And they are still young enough to be socialized with people. If they've got older birds already, the chicks can go straight to the grow out to begin getting used to the adults.
I can get $10 a chick for them at that age. Around here, I'd be lucky to get $2 for unsexed chicks.
Just 3 pullets can pay for feed and the cost of incubating and brooding.
Same can be done for ducks and turkeys. Or you can try raising game, like pheasant.
For rabbits, you will have to do a bit of research. Are you in an area where pets are more popular or is it meat that people are after? This will determine which type you raise, and how much you can get for them. One type of rabbit that is really in demand, nation-wide, is fiber rabbits. Angoras can cost quite a bit to acquire, but you can sell the wool online, so you sales aren't limited to your local market.