I know Maplin have one for 39.99
Here in the U.S. we can get them a little cheaper. I usually see them for $40 - $50 USD. Amazon.com has one for $30 USD. Around Haloween, I think I've seen them for $20-$30, and on Haloween day, or the day after Haloween, they usually sell the Haloween left-overs for half-price. I actually tried to buy a "left-over" one, last Haloween. But, when I went to the store the day after Haloween, all of the Haloween merchandise had already been replaced by Christmas merchandise.
I've experimented with an Ultrasonic Mist Maker
, but even with a medium-size mist maker, I didn't get enough fog. I wanted to give it a try, because they are low power, and since I've got about 1000W of lighting, I might blow a circuit breaker if I add a 400W-800W fog machine. Plus, the mister runs off pure water. Too bad it didn't work...