IMHO, there is no need for an air ride seat in a cab over truck. In fact, I hate the way air ride seats "ride". I feel like I am on a bungee cord or a bouncy house. A properly tuned one with a damper would be a huge improvement, but I must imagine those are even more ridiculously priced, if even a monster were to exist.
I have formulated a plan to build a suspened base at a friend's metal fab shop and use an adjustable shock from the rear of a full suspension mountain bike. They can be had in air or coil-over, with plenty of adjustments.
The simpler solution and what I have done in the mean time (read: most likely permanent) is to spend some time in a wrecking yard and find a seat that you like. You could also take a page from the rock crawlers book and use a suspension seat like Mastercraft or PRP. They are very comfortable to trail ride in all day, crawling over rocks the size of a Mini Cooper and riding through the desert in the fast whoops, etc. No need to re-invent the wheel.
Here are some mock-ups that I did. One seat is from an E24 BMW (rear seat, SUPER comfy, but can be expensive and somewhat difficult to find.) Second seat came from a Toyota sports car of some kind, maybe MR2 or Celica GT. Toyota usually has pretty comfy seats. Honda sport seats are comfy to me as well. I have driven 28 hours straight in them with little to no fatigue.
These were mockup pics for level, proper height in cab, etc. Factory brackets were cut off and a simple L bracket was made to match the holes in the Fuso floor. Thread bolts back in and done. Adjustable for height, slide and tilt and significant improvement over the awful factory seats. At the end of the day, you are sitting over the front axle, so seats are only going to do so much. Proper suspension tuning (no $7,000 leaf springs required), along with good shocks and correct tire pressure for your driiving and terrain will go a long way towards improving the ride.