Edit - I meant <$100 - can't edit the subject...
Finished this chamber last weekend; it easily fits 2 carboys. Could also fit a couple of cornys/1 gallon jugs. Could probably fit 3 5-6.5 gallon carboys, but I haven't tried yet (only have 2 currently). I didn't take many build pics, it's pretty self explanatory. There's a space heater to heat and a fan mounted on a dryer vent to blow cold air in from the basement (year round my basement is 50-60F). This should work great as I mostly brew ales - when I want to brew a lager it'll either have to be winter or the kegerator will have to be used during primary.
Dimensions - 32" Wide / 24" Deep / 32" Tall
Materials for chamber:
$6 - 1/2" Culled Plywood (cart at HD with cheap wood) 3 48"X48" sheets
$8 - 4 8' 2"X4"s
$10 - 8'X4' R3 insulation 1/2"
$3 - Pair of hinges
$2 - Pair of Hook/eyes to hold door shut
$3 - Foil tape
$18 - space heater
$2 - dryer vent
Already had CPU fan/PC charger and screws
$52 - Total for the chamber
$7 - project box
$10 - 12 GA extension cord
$25 - Temp controller (ebay)
$42 - Total for the temp controller
I figured I'd just post pics of my brewing storage & kegerator (which I initally made for lagering before I had a kegging setup) while I took pics of the chamber since it's all right next to each other.
Edwort's Apfelwein & BCS American Brown ale (1 gallon oak matured mead behind the Apfelwein, barely visible - it's about 9 months old now). You can see the dryer vent on the right.
The kegerator fits 2 pin lock kegs and the C02 tank, it's SO close to fitting 3 kegs, with thinner insulation and staggered properly it just might fit another.
Let me know if you have any questions, hopefully this will help to show others how simple it is to build a fermentation chamber.
Credit for inspiration: