my tuppence....
I'm not knocking people for going for the cheaper alternative, but what I would say (at the risk of being a bore) is that the Airhawk is made by a company called ROHO, and it was originally designed as a pressure relieving device for wheelchair users. It therefore has to meet extremely high manufacturing standards for durability and infection control amongst others, which would account for part of the high cost of what is effectively re-branded medical equipment, ie the Airhawk.
Additionally, the ROHO design is patented because it cost an awful lot of money to develop, and it is hugely effective at reducing pressure areas in wheelchair users as a result.
From a clinical point of view, there are two main causes of pressure areas, uric acid (hopefully not a problem for a motorcyclist!) and sitting in a fixed position for an extended period, which is what any biker with a numb arse is experiencing.
The effect of the latter is to restrict blood flow to the area, creating a buildup of toxins in the cells, and causing the nerves to allow reduced feedback. Add into this shearing forces and vibration issues and you've got the perfect recipe for discomfort and ultimately, pressure areas, also called pressure ulcers.
I should add that I'm not affiliated to ROHO in any way, but I do manage an NHS wheelchair service (amongst other things) where I was also a clinician, once upon a time....
Point of all this being that even if you cure the numb bum, you are still sitting in a fixed position for extended periods, which is not ideal from a physical wellbeing point of view, and thats before we get onto our legs and what its doing to them.....
Little and often I reckon...