$30 or so in plants
$10 or so in substrate
$10-15 or so in decorations
You may be able to find deals on these on craigslist/ebay/here.
You want places the puffers can hide, and substrate the plants can actually root in. If you choose fake plants, you might be able to get away with a bare bottom tank, but keep in mind with messy eaters it'll look bad fast if you don't keep it clean.
A heater and a filter. $10 for a sponge filter, $5 for tubing, $15 for the air pump. Alternatively, a HOB plus media is what? $30? I'd go with a 20-40 gallon HOB for puffers. Gravel vacs are $20 or so. You'll want one whether or not you're using substrate. Heaters are $15?
Hood plus light, $34. I just got one yesterday.
This isn't including food, conditioner, medicines -- pea puffers are largely wild-caught and come in with parasites much of the time.
All in all for a ten gallon, I'd say $100-200 just on set up. I spent $107 yesterday getting stuff to set up a 20 gallon. And we had about half the stuff we're using already -- we're not using live plants, and we have substrate and some decor.