Practically speaking, I think a $20 U-lock will have the same effect as a $100+ lock; either a complete deterrent, or none at all.
I'll differ - $20 U-locks typically can be broken with manual tools like hacksaws and bolt cutters, whereas more expensive ones (even the $45 Evolution) typically can't.
(I'll also note that two forumers have recently posted about having bikes stolen using the $30 Kryptonite Kryptolok U-lock)