Bicycles get stolen because of the quality of the lock, not because of the quality of the bike.

Pretty much every lock under $40 can be cut thru by inexperienced criminals pretty quickly, while locks over $100 are so tough to get thru that even experienced thieves will tend to move on to an easier target. 

Mid-priced locks are, well, somewhere in the middle. Buy a good lock for urban areas. Don't let saving a little cost you a lot.