Don't know what the conversion is for jetfuel, but it looks like 1800 lbs of water would work out to 215 gallons. At a 1.50 per gallon that would only be about $325 worth of fuel.

Looks like the pilot came up with a creative and reasonably cost-effective way to get the plane within the weight parameters (although this doesn't include the costs related to a whole plane full of people's time.)