You could ask at the store about those sales, I didn't know they did anything like that myself. 50G tanks are expensive, I know... A 20G will be ok. I was just saying what I feel would be best for them. You might consider turning the 20G on end to give them more up and down room, which they need even more than side to side room. Just make sure when you get the new frog, you quarantine it for a week or two to make sure it is healthy before you put it in with the frog you already have. Fake plants are fine too, I just prefer real ones because of the benefits, plus I feel that it is much less maintenance with real plants. You can also mix fake plants with real ones, adding real plants as you are able to. I totally understand herping on a budget, lol...
I do the same myself. One other thing to think about is that it is nearing the end of summer, and alot of places are clearing out their stock of plants, so now is a great time to find really good deals on plants. Don't be afraid to buy small plants either, because they will grow quickly in the conditions you will be providing for your frogs. Oh, and make sure you're having fun with it...