A 130kg friend of mine whose ST1100 has a standard seat, uses a doughnut purchased from a camping store for just a few dollars, puts one puff of air into it and tells me that he can ride all day in comfort.
He's ridden on an Airhawk before and says that he's more comfortable.
If you can put up with unjustified jokes about hemorrhoids from your mates, it could be a cheap alternative.