To follow up, I think this might be what I'm talking about. You could use two of those to create the connection, or if the building at the other end is small enough and the signal reaches everywhere you need it, that dish might be all you need for an access point.

I've never worked with these devices before, but they can easily cover the distance you need (they're rated for 30km, though I doubt that's real-world), and they're cheap. You just need line of sight for the two endpoints.