Hello and welcome to FF! With that budget and the "semi-active" requirement would think Fire Belly toads would be good. Right now Petco has a $ a gallon tank sale and you could get a 20L for $20. Here is a care article for them: Frog Forum - Fire-Bellied Toad Care and Breeding - Bombina orientalis and relatives and another one on how to choose a frog: Frog Forum - Choosing a Frog .