What lounge are we talking about?

The non-schengen KLM lounge @AMS can be accessed with a Sky Club membership, $90 for 30 days. Not a bargain if you don't need it otherwise, but probably a lot cheaper than a Y ticket if you really don't need the flexibility (unless is it someone else who's paying the ticket). You'd also need to visit an actual Delta Sky Club to pick up the card.