Probably a lot less. One can effectively use a small aquarium blowing pump by connecting the sucking tube to the air intake instead of the output, and they're cheap, like 10 bucks each. The electronics would be trivial, and the switch could either act on a solenoid valve (less than 10 quid) or be a small hole in the side of the tube so that closing it with a finger would let all air in through the end of the tube.