Things in airports are generally more expensive than outside an airport. In the States, most gyms cost from $10 to $25 for a daily pass. In Europe, it's 15-35 EUR in my experience. (Airport showers alone are also usually a minimum of $5-10) I wouldn't expect an airport gym to offer entry for less than $15, and considering lounge access goes for $35+ for an outlet, wifi, and snack mix, $15 would be a bargain.