It can be as simple and ugly (a plastic storage tote with styrofoam "island" supposedly worked for someone...) or as brutally extravagant and expensive as you like. I personally rock a 10G ($10 during Petco's $1/gallon sale) for 2 females and a male. Just some cheap fake plants (Walmart.. think I paid about $10 for 4), a secure lid (forget cost, you can make one... just make sure it either has locking tabs or put something heavy (for a frog, not you) on top, as they can stick to the glass and climb out through any opening they can get their head through), a small intank water filter (was about $10), and smooth aquarium gravel (don't recall the price). Some say not to use gravel as they may eat it and die (from impaction, or not being able to poop), but I have a small stainless steel bowl ($1) that I put their food in (Dubia roaches and occasionally wax worms. Crickets will jump out of the bowl). You can skip the water filter, but you'll end up having to change the water more frequently. I change mine about 75% every Friday (that's about when it starts to smell a little if I put my face over the tank). Your highest cost will be food (over time, as they don't eat anywhere near as much as say... my bullfrogs or bearded dragon). Many people recommend vet checkups, but it's unlikely your local corner vet will be of any use. You need a vet that specializes in reptiles/amphibians/other exotic pets and they mostly stay near large cities (not only is it unusual to own these types of pets, but most people don't see a vet). Personally, there isn't an exotic vet anywhere near me, so I took care to select the plumpest, most active specimens (doesn't apply to my American toads or bullfrogs, as they showed up at my backdoor). Generally speaking, unless it's already sick when you first get it AND you feed them properly AND keep their home clean, most juvenile-adult frogs will live for quite some time (I'd shy away from babies, as frogs will lay a LOT of eggs. They do this because many of the hatchlings don't survive).

This is not a complete list of what you may want and others may disagree with what they NEED, but it's worked great for me. Aside from some sickly specimens I foolishly took in (a pacman frog from Petsmart that lasted about 8 hours and a juvenile bullfrog about 1/3 the size of a normal tadpole, etc), I've really never had any die on me (gave them away before they got old). Of course that will change now, as I am self-sustaining and not going to live places where they aren't allowed. I suggest you check out the firebelly subforum and google care sheets for a more well-rounded analysis.