In small quantities, bike tube patch kits aren't terribly expensive. However, if you're riding all year, stocking up on kits gets expensive. If you don't mind getting your hands dirty, Instructables user Zaphod Beetlebrox show a simple process for making your own patch kits in bulk.
The Instructable is pretty simple and essentially just takes the common parts of a patch kit and comes up with the DIY equivalent. All you need is a little contact cement, sandwich bags, sand paper and an old inner tube. With all those in tow, you simply need to cut everything down, seal the contact cement in a bag, and you're done. They're certainly not as nice to look at as a store-bought kit, but it doesn't really matter when you're out on a ride. Head over to Instructables for a full guide.
Make Bulk Bicycle Tube Patch Kits [Instructables]