Tiger-legged monkey frogs usually go for around $25 on average. They are a very good place to start.
Your 20 gallons would be good for red-eyes, or better yet White's treefrog, Litoria caerulea. White's are a good pet frog, but they will need more room then you can presently accommodate. They are not as colorful as the tiger-legged monkey, red-eyed, or clown treefrogs but they have outgoing personalities and tolerate handling better than most frogs. They are readily available, and I see them all the time at Petco. They are probably the best first frog, but again they need a little bit more room than the previously mentioned treefrogs. They are also known as dumpy treefrogs, as some of them can get quite fat.
Here is a pic of one of mine. I have two of them.