Glad you came here first before going to get the fish! Here's a basic list of stuff you should get.
Water conditioner (Personally I like Prime - ask for it though because it usually isn't in the betta section)
Betta food - Hikari pellets are good as well as bloodworms (I use freeze dried, but some people prefer frozen) Your two major concerns here are - don't over feed him (2-3 pellets once in the morning and once in the evening is pleanty. I usually skip one day of the week so their systems can clean themselves out)
Betta home - this is where you will here people say a varity of different things. 1 gallon is the _bare_ minimum you want to keep your fish in. However a nice 2.5 gallon tank won't cost you much more.
Thermomter - Bettas are tropical fish. If the water does not stay above 74 degrees on its own you will probably want to get a heater
As far as the filters go, if you get a 2.5 gallon tank and do weekly water changes, you won't need to filter it.
Now. The betta itself. I would suggest letting her go pick out the fish she wants. Also, who will be responsible for water changes? Who is going to be responsible for making sure mr betta is fed correctly. You'll want to change his water once a week.
I'd say you'll probably end up spending somewhere between $20-$25 on everything.