The valve blocks on these are fully servicable. The people who charge you for a whole new one are doing so because they don't know how to fix them. The lines and fitting are standard industrial pneumatic components. If you're having compressors die, you've probably got a leak which is causing them to run over-time.
Best advice, find someone who knows how these systems work and use them to maintain the vehicle. The parts (airbags, fittings, lines, seals etc) are not expensive.