I went into my local Superdrug the other day (I live in Manchester) and they had airbeds on offer, I cant remember how much they were exactly but i think the double one was £7.99.
That's a good price, if it stays inflated!... In my experience, the cheaper airbeds aren't very reliable and can lead to an uncomfortable sleepless night... You might be better off spending three times that price, for a double 'Coleman comfort' airbed, and a guaranteed good nights sleep, it's definitely worth the extra money!...
I bought one from Superdrug last time they were on offer, it was just for my daughter to sleep in the garden in her little tent in (I said i would never go camping) but got the camping bug lol and have been using it all summer and it is still going strong, bought a single one too and that is still brilliant.
We got one of the Tesco pricing error bargains earlier in the year where we got a double airbed plus an electric pump for £3.01! However, I quite agree with Andy Pandy - the bed was so uncomfortable that we've just purchased the Coleman Comfort. Nevertheless, we still got a great deal just for the pump!