As per title I'm looking for a decently sized airport that charges 0 or close to it for its slots. I'd like to run a small plane operation and keep the costs to a minimum...
Well, it all depends on the game world, since the costs at airports are dynamic and based on the number of traffic it has.
Said otherwise : whatever the base, if you fly all the slots, it will end up very costly for the last ones.
That being said, IIRC, if Visby is not overwhelmed, it's a kickass place for flying 19-seaters to all Sweden. It has 23 slots per hour in 2028's GW3, and you can probably cover all Sweden with 12/14 small planes. You're not going to be overcharged in slots - or landing fees - over there.