According to the site:
"PAY WHAT THE TEMPERATURE IS
25TH FEBRUARY - 10TH MARCH
The weather today: 11°C
The price today: $11 Just before having his tea, the monster goes out to the weather station in London to check today's temperature. He sets the price accordingly and tweets to his friends from @AGoodSnowman."
Interesting promotion strategy (lowest temperature in London during that time frame is supposed to be 5 degrees)