In petstores near me they sell for about $75. From breeders they are about half that, and much healthier. Also a breeder will usually be willing to help you out in the beginning and will actually give you good advice rather than try to sell you on everything they have and rip you off like most petstores (I work in a petstore, I know) Some petstore associates will be extremely knowledgable and a good resource, but always double check the info you get from them. A 10 gal is okay. Personally I would start in a 10 gal and get a 15 gal when they are full grown. This site has a care sheet on leopard geckos with the basic essentials but it helps to do a search and read as many different pages as possible and compile a list of things you need to know. As well, with any pet, it is usually best to pick up a book from a petstore or check out a few books on the animals you are looking into from the library. Do some research, take some time setting up the tank really nice then go out and get your animal. Sometimes there are Reptile shows wich are an awsome source of knowledgs as well as a good place to meet breeders and find one near you. Many breeders will also sell some of their stock at these shows.
Good luck