It's some of the cheapest flying you can do, especially at a club. You pay for the tow and the ship time but there's no fuel and no engine to overhaul.
People flying in aeroclubs here in Czech Republic AFAIK not pay for flight time (they pay fowlr tow of course) - they pay some yearly membership fee & slave for the club during the year doing maintenance and repairs or running information service on the aeroclub airport tower.