I've done my last 1,000km on an old second hand bike that cost me $100 (plus $100 in upgrades, though even those were optional). If your current bike isn't suitable, why not see what you can pick up used from a garage sale or your local thrift shop?

Although I certainly appreciate the beauty of a high-end bike, don't let a lack of $$$ to spend stop you from touring.

http://travellingtwo.com/resources/the-100-touring-bike