Bikes are great things. Unfortunately, no bike lock is going to stop someone from stealing your bike. Locks are only deterrents that make it more of a hassle for someone to steal your bike. This article from Slate magazine
will show you how much you need to spend to protect your bike from hand tools. The only ones which stood up to those tools were at least 90 dollars and can still be broken in under a minute by inexpensive power tools that real thieves know how to use.
With that said, you can save some cash by making your own deterrent out of some chain, a padlock, and (optional, but very nice to have) some cloth. It will provide similar security to many cheaper bike locks and could save you 10 to 20 dollars in the process.