Tim,
These guys are on target with the suggestions for running Cat 5/6 wire. Its easy work with, and you can get everything you need at Home Depot. 200' is pretty long cable run, so I would go with Cat 6e. I don't know if they make Cat cable with jacketing for underground burial.
Another option would be a point to point wireless setup. Instead of a tradition setup with a single broadcast point, point to point setups are designed to jump across gaps. The skys the limit on price, but you can find them relatively inexpensively.
Here is a kit:
http://radiolabs.com/products/wireless/point-to-point-bridge.php
If you are willing to use used equipement, we might be a able to put something together for less. I've only worked with Cisco products, but they are a lot of $$$. Can I assume you don't want to sink $1500 into this?
