Switch him to a tub. Can't get cheaper than that.

If you absolutely can't do without a tank, you can try to find something off craigs list that you can afford. But you'll have to disinfect it carefully. You can really do up tubs nicely though and they hold humidity much better. Having said that, I do use a tank for my BP and I like the Zilla Critterkeepers. A 20gal would be a good choice for a long time and it is going to run about $80 but it comes with a locking screen lid which is very nice. But I still recommend a tub....