lots of time and money, its not really a business but an expensive hobby. Sure some, a very few make an OK amount of money but they have the top $$ high end newest morphs and the regular hobby fav's.
If you break even on feeders and can buy a new leo from time to time you are doing well.
First You need to make sure you can aford it, have time for it, and Have knowledge on the geckos you are keeping and seeling.
You will need proper housing for every single gecko. You will need an incubator to hatch the eggs of course. You will need to find out all the regulations in your comunity such as what permits you need and such.
You will need to have a vet that is Knowledgeable about Herps. You will need a website or post the geckos you have for sale in the classifieds. You need to learn how to properly ship geckos. There is alot to a business and it also depends on if you are breeding on a small or large scale. Do some research ask questions and decide if it is right for you.
I would start out by finding out if you need a business license where you are planning to start your business. Some places do not require you to have any such license and you can sell to your hearts content as a business and out of your home. However, if you do live in an area that requires you to have a business license; then you will most likely have to find out what's expectable if you are going to run your business from your home. In most cases the product can not be in or attached to the home and you will need a unattached building to set up your breeding in. Also, with a lot of in home business they can not be open to the public if it is in a residential area like a neighborhood. Another thing you will have to find out is if there is any restrictions towards reptiles or the number that can be kept on the premises; call your local animal control officer for that info. In my case I can run a business without a business license out of my home with a in home office, but I can not have the product in or attached to my home if it's being bred. A separate building is fine as long as you do not have a covet that forbids it. Some places might even require an inspection of the premises that is going to be used as a business. We have our main breeding building an hour away in another county where I have family that helps with it. Interesting enough, I can keep the product on the home premises for up to 48 hours with the intent of shipping it only. I also can except shipments of the product too. Absolutely crazy! Another oddball thing is that within the same county two cities that are side by side can have completely different requirements. Our sister city doesn't care if you have a business license or not or if you run it out of your home or not and they are the bigger city too!
The second thing I would do is take down prices for all that you will need. Starting with several books, rack system/ systems that are big enough for housing the adults and the offspring - either one built for you or a list of supplies to make your own - keep in mind that you might get eight viable eggs per female, heating, temperature controllers, tubs, supplements, liners for the cages/ tubs, hides/ egg laying boxes, egg laying medium, incubators if you need them, incubating containers, incubating medium, markers to mark the outside of the egg containers, a system to keep records and any supplies that will be require, a very good gram scale, batteries, a camera, on hand meds and supplies like syringes, a good vet that has some basic knowledge of reptile care and interest, cleaning supplies that actually disinfect without harming the geckos, feeders, food for the feeders, housing for the feeders, any supplies that you will need if you decide to breed your own feeders, all the shipping supplies you will need including heat packs and cool packs, an account with a shipper and possibly approval to ship live harmless reptiles, a website helps, accounts set up at all the main classifieds, possibly the cost of per booth as well as gas and logging if you decide to do the reptile shows, advertising, how much the total amount of electric, water, and etc, for a given month to a year to find out how many geckos you will need to sell to just break even, then the selection of leopard geckos and how many you will need of any particular morph - the cheaper the morph the more you will need to buy to breed to make any money from them as a general rule. Start up costs are always the biggest lump of money and you will more than likely never make that up. A good credit card wouldn't hurt either for emergencies. You will also need to find a good CPA that knows how to handle your type of business. An insurance company that will cover your type of business really helps if you are going to be big. I'm sure I have left out something but I am really too surprised of how long and costly it really is! Now I know I must be insane or I really do love those geckos!
Of course, you could do what a lot of us do and start out really small like a hobby and just grow up to become a business. I would because it's more interesting and fun. It also gives you the opportunity to judge it as it grows. You might find out that you do not want to be a business as it really is a lot of work, time, and money. Start out by buying a good book like Ron Trempers big book 'The Herpetoculture of Leopard Geckos', In it you will find a section on page 146 that briefly outlines different types of ways to approach the leopard geckos as a business or as a hobby. You can find this book either at www.leopardgecko.com or at www.amazon.com. It's a very good starting point as there are many things to consider.