I'll stand to be corrected on the proper rating on helper spring.
However there is such a thing as cheap airbags. You can get bags and the install kit for sub $200, its nothing more than the bags and a valve stem style connector for pumping them up. Real cheap, super thin walls on the airbags and will blow out the first time you hit a bad set of tracks with them pumped up and under load.
As for Semi bags, Firestone has a kit that will mount single height bags. You can go and get upper frame mounts for bags, and retrofit the lower bag mounts in order to fit single height bags. I never said you'd get standard double heigh bags under there, but Firestone does sell low height bags for the exact use on pickups. I'm not talking the sleeve style, yes those are only for leveling, it could take 75psi to level 1000lbs, but on a small bellows style you'd need less than 40psi.
There are plenty of pickups with heavy duty airbags, its a little more expensive than buying new spring packs plus coils for the front, but it will be far from a horrible ride. Air suspension provides a very smooth ride, and only increases stiffness with load, exactly like helper springs. Semi's only ride hard because of the massive spring packs and high tire pressures they run even at no load.
And yes, bags are not solely used to hold weight, they are used for leveling tractors. However they can take 10klbs load to do so.
We can agree to disagree bcboy.