I am sure you will find international airports where you can do it for under ten quid, but I can't think of one off hand.
I was tempted to suggest you check out Bulgaria (not a favourite Czech communist party liason posting in the 1960s; a toilet was a hole in the ground)
but I happen to know Sofia has a bill longer than your arm, matched only by Tirana whose 200 euro + bill includes such wonders as runway lights.
So, if you do find one, will you want to go there? You well might; wherever it is, the local girls will be excellent value.