I'll tell ya' what i've learned in the few months i've had bettas.
At LEAST get a one gallon bowl ($5) for your betta. What a lot of people do is buy a ten gallon tank ($10) then seperate it into 4 or 5 sections.
NO PLASTIC PLANTS. plastic plants and jagged gravell can and will tear up a betta's fins with no remorse. Silk plants (approx $7) or real plants (Approx. $4) are your best bet.
Cover the top of your betta's bowl or aquarium. Sometimes the fishies like to experiment with living on dry land. that's never a good thing. Though they DO breath surface air though, so make sure you leave space on the top for them to come up for air. IE: none of those stupid plant-betta 'eco-systems'.
Betta pellets work well ($2), suplimented with Freeze dried bloodworms ($2) will keep them happy.
Aquarium salt (about $2 at your LPS) will keep your betta's immune system healthy, and thus keep your betta healthy. Beware though, not all fish can thrive in brakish waters, so if you are planning on keeping Cory's or other fish with your betta you'll want to avoid aquarium salt.
Bettas are tropicall fish so a water heater ($5) would be prefered and probably appreciated. Not to mention, without it you'll end up with a sluggish unhappy betta. no fun to watch at all.
and that's about all i can think of. The prices are just based on what i've gotten. Wal*mart has very good prices on tanks, decorations, gravel etc. and will be notably cheaper then your pet store. But i wouldn't recomend getting your fish from Wal*mart, see if there is a local breeder in your area so that you can be gauranteed a healthy happy animal. (( .. heh.. comming from me, who's every fish was bought at a pet store. lol. i'm just a glutton for punishment i suppose. ))
Hope this helps a little!
Oh yeah! also, check out this pinned topic, it should be of help!
http/www.fishforums.net/index.php?showtopic=32461