Probably the cheapest solution would be a 20 ma current loop circuit. 1 transistor (PN2222A), 1 opto-coupler (we use a 4n26) and a few resistors and you are set. I think I remember something about that being good for around 2000 feet, especially if you can go with a lower baud rate. Having said that, I would probably still go with RS422/485 just because it is more of a standard these days.

David