That is 40,000 admissions to the event. Then there is all of the Thomas toys that get sold. And the refeshments, etc. Even with the fees paid for the engine, the fees for the use of the "Day out with Thomas" name and images, cost of merchandise, and the other costs of the event it seems to be a money maker for the sites that that have done it.
The words of caution I would give is that the site needs the facilities to deal with the crowd and have a population base within convenient driving distance.
There has been a number of threads about Thomas and the Thomas events posted on the Railway Preservation News "Interchange" discussion board.
Brian Norden