Eric, first, do not even consider running the network leg above ground. I have done it with RG-58/AU and it was struck by lightning. I am lucky that no one was hurt. Both computers at the ends of the leg were destroyed. If you run RG-58/AU below ground, one end (only) must be properly grounded. If both ends are grounded, a dangerous ground loop situation can develop. A thinwire network can be up to 300 meters long.
If you already have a below ground telephone wire running between points A and B and it is not part of a PBX system, you may want to try a phoneline network (http://duxcw.com/digest/Reviews/Network/dlink/dhn910/dhn910.htm, http://duxcw.com/digest/Reviews/Network/intel/anysum.htm, http://intel.com/anypoint/index.htm?iid=oldhomehtm+meta) . It may stretch to 600 ft at 10 Mhz., but may run slower depending on the condition of the wire and ambient noise. The wire should not be in the same tube as power cables. Considering the low cost, I would give this a try if the wire were in-place.
Another possibility would be a fiber optic leg. I have not done it, but from what I have read it should be immune to interference and should not be a lightning hazard. Looking at the Black Box catalog… the PC boards are under $300 and the cable would cost around $400, but I really don’t know everything about it and what else is involved/required in/for doing it.
The Intel wireless network is spec’d to go 1 to 11 Mhz, 400 – 1,500 feet. See http://intel.com/network/products/wireless.htm.