I bought two bathroom vent fans from Ace Hardware, which are rated at 50 cfm each. Then I bought a clear storage tub from Walmart that was big enough to cut the bottom out and put both fans side-by-side in it. I secured and sealed them to the tub with duct tape. I bought an 8' long 3" wide aluminum duct tube that fits the fans to vent them out my window. Keep in mind, you don't want to have a long distance for your air to travel to get outside. The shorter the travel distance for the ducts, the better your booth will work. My booth is right in front of my window, so the air only travels about 1 foot out the window. You can't just put the ducts out an open window, or the wind could just blow back the fumes. I opened my window halfway and used old cardboard to fit in the space with 3" cutouts to fit my ducts. When I'm done spraying I can easily remove the cardboard and close my window. If using two fans, you can either use two extension cords, or if you are electrically minded, I'm sure you could wire the two fans together. The fans make my spray booth a little back heavy, so you have to secure the front down somehow with weight or tape or something so it doesn't tip back. Lastly, you will need a furnace filter to catch the paint particles. I would stay away from the really cheap ones, as I think they would let paint particles pass through them too much.
Also use paper towels or newspaper to cover the bottom and sides inside the booth for easy cleanup.
Some people might think that my combined 100 cfm fans are not strong enough for a spray booth, but with my short ducting distance they work great. I can open a bottle of Tamiya extra thin cement and place it in my booth and not smell the fumes at all. That being said, I still recommend using an organic filter mask like the one that 3M makes.
Here's a list with approx cost:
Storage tub: $8
2 Bathroom vent fans: $32
Ducting: $5-10
Ducting clamps: $4
Duct tape: $4
Furnace Filter: $7
Extension cords: $5-10
Approximate total cost: $75
OKLAHOMA where the wind comes sweeping down the plain, and blows your house away in the month of May...
On the bench: Revell B-24D Liberator