I think they said 15 tonnes (assuming they're talking UK weights, being British and all), which is 33,000 pounds (approximately). 1 ton = 2204 pounds (UK). 33,000 pounds is about 4860 gallons. (6.79lb/gal). Assuming that the airline is paying about $3/gal with a good contact, that's $14.5K for the dump, and another $14.5K on the ground at CYYZ to refuel. So not exactly cheap, but cheaper than running off the end of the runway 'cause you're overweight on landing.
Just part of the cost of doing business, I suppose.