You could do an ARS-5040 and just a couple levels, they are $212 per level, so about $50 per tub and that includes the metal heat panel too. You can add onto it as you go. I use this size for hatchlings all the way up to about 1500 grams, I keep my males in this rack too since I keep them a bit smaller. Plus it stacks on top of a boa tub if you ever go that route. I actually have two boa tubs for my dwarf retics with ARS-5040 levels on top.
http://arscaging.com/ars5040_hybrid.html