While I did not tajke advantage of the ounge when we were at SJO in August, I did notice a "Salon VIP" on the lower level -- its access is right by the "food court."
I do not remember the exact details and couldn't find anything online in a quick search, but I do recall the setup being similar to what
ricktoronto mentioned for MGA -- I believe it was $25-35 with drinks and hot entrees included.
The
website of the airport operator isn't working properly for me, but I imagine it has some details about the lounge too.
Unless you're fixin' for US franchises, the dining and entertainment options (other than free wireless) are pretty non-inspiring at SJO, so the lounge might be a good investment if you've got some extra time.