A five gallon tank is probably large enough for them to comfortably breed in, but unless you have very few fry it's probably not going to be adequate to raise them all until they're ready to be jarred. I go with one ten gallon for breeding, and have a spare in case the clutch was unusally large so I can accomodate their grow-out period. For some good advice on exactly what you need to start spawning, please check here:
http://bettatalk.com/spawning_supplies.htmAs for cost, most petsore bettas with veiltails sell for around 3.99 to 4.99 USD regarless of color. Sometimes crowntails will sell for perhaps a dollar more. Of course, like all fish that have a well developed fancy and very many distinctive varieties, you can pay a lot of money for a particular patten of betta fish with full finnage. I'll assume that these are not what you're looking for ^^. If you just want some nice petstore betta fish, then you'll find they cost between 3.00-6.00 dollars, depending on where you buy them ^^.