If considering breeding, there are a lot of things to think about:
+ Unmatched bettas (those with different tail types) or bettas that you don't know the heritage of (so where you don't know about their parents/grand parents) present a huge risk of giving you "mongrel" fish that don't have desireable traits. These are very difficult to find homes for and you may end up getting a higher rate of "culls" (bettas that are too sickly or deformed to be sold and may require being euthanised).
+ You cannot house the males and females together and expect them to get on with it. It's not like breeding guppies or something whre you leave them un a tank and reap the rewards when they decide to get jiggy. If you put them together before they are ready to breed they are very likely to either fight or become very upset and stressed from being kept together. They are solitary animals.
+ The set-up could end up being quite expensive. You need a spawning tank (10 gallons should be fine), a grow out tank (a 3 foot tank is best as it give the fry room and hopefully will stop them getting stressed from being crowded) and lots of small tanks, tubs or jars (at least 0.5 gallon each, IMO) for the males and feisty females as they start fighting. Everything needs to be heated (so you'll need a very hot room or a heating system that can cope with multiple tanks) and you need to have a method of filtering each tank/tub or some other way of keeping the ammonia down (so daily water changes on potentially 100 jars or tubs).
+ It is a lot of work and a lot of mouths to feed. You need dedicated fry food and live food cultures for something like baby brine shrimp. You'll need to dedicate maybe a couple of hours a day to water changes and general maintainence. You'll probably lose money because selling on the babies can be very hard and you might not meet your costs.
Don't go into it without an excessive amount of research. Join several dedicated betta forums, find local breeders and ask them how they do it, look online for breeders and e-mail them for advice. Ignore people that say it's "easy" and you just "put them together and let them get on with it". However if after a considerable amount of research and with a clear plan for housing the babies and finding homes for them when the time comes you still want to give it a go, good luck.
Remember - prepare everything in advance, even the stuff you might not need for a month or so. If you wait until the last minute and don't manage to get something sorted in time, you're just failing the little creatures you have helped bring into the world.