you can get a good used NA engine any where from $500 to $1500. I said this by experience, I was in the same situation as you were, I did some research called some people and end it up finding one with 80K for $550 shipped! The engine came pretty nice and clean, I was a little sketchy about it at first but after installing it I ran some compression test and the engine was solid. Check the spark plugs out when replacing them and they were in good shape also.
Local Junkyards will want at least 1K for an used NA