Thanks guys.

AV, I'd be more than happy to help where I can.
The exhaust is 4" all the way through. It's comprised of thin gauge flex duct and 90's, duct tape, aluminum tape, 4" ABS couplings and blood.
There is a 120mm fan that moves the air. The 4" ABS couplings are attached to both sides of the 120mm fan. I attached them with 1" x 4" plates of metal (times 4 on all 4 sides of the fan). You could probably attach the couplings with straight duct tape to the fan, anything to make a seal. If you decide to go the 120mm fan route, make sure your using one that pushes some decent CFM, most are low power/low CFM.
All of this pushes into a scrubber. Scrubber uses kitty litter crystals instead of carbon and the plans to build that can be found in the faqs section.
You'll have to ask if you need more specifics on anything else.

Costs? eh, I did my best to NOT keep track. But I would guesstimate, just on the cab material and venting, that it was at least a couple hundred bucks. Wood, screws, duct, fan, hinges, paint, more wood, tape, insulation, etc. You'll need a hole saw (4") and a powerful drill to get through the walls of the cab.
Oh! There is a 2" and 1-1/2" ABS intake as well.