I posted this over on TLB a while ago - basically the Leisure Mat is a commercially made balloon bed. Unfortunately there's no web-site and I don't have a digital camera, but I'll see what I can arrange ...
And just to note - that weight of 12.7 ounces/360 grammes is for a pad that's 31 inches wide versus the 20 inch Thermarest.
Was browsing in the supermarket here in Tokyo today and came across the "SunFriendly Leisure Mat". Cost was 980 Yen - about US$9.00.
Basically it's an air bed with a top made of the aluminium foil used in emergency blankets - blown up it's 184 cm long x 80 cm wide x 3 cm deep and weighs 360 grams. If it was 51 cm wide - standard Thermarest size - it'd be about 230 grams.
It has 60 tubes that run width-wise rather than lengthwise. The tubes are connected to each other by four gaps but the air doesn't move that easily between the tubes, meaning that the tube your weight's on doesn't completely flatten out.
There are two valves for inflating it, which wasn't as hard as I thought it would be.
I tried it out in the house and it's actually very comfortable, if a little crinkly.
Durability? Well .....
I recently bought a 40+ litre pack designed with a rear mesh pocket so that you can put a sleeping pad down the back, like a Mariposa etc. It's meant to be for earthquake kits. Usually sells for about US$40, I got it for about US$25 using membership points.
I'm thinking of doing an ultracheap overnighter ...