What's the most secure type of bike lock that costs less than $30?
Honestly, I must say that there is no such thing.  Sometimes you can find a U-lock used on Craigslist or similar websites and you might get a better deal.  Otherwise, $30 is below bottom-of-the-ladder for U-locks.

And think about it.  What are you paying for with that 30-50-even $90?  It's a one-installment insurance policy on your bicycle.  I would suggest that you lay down some real money and get a good lock.  If you don't, you can either


Never take your bike anywhere that won't allow you to bring it inside with you.
Plan on replacing your bike frequently and encouraging the bike thieves.


My frivolous answer:  Really.  Maybe you shouldn't spend a bunch of money on a bike lock.  If you get a cheap lock, the bike thieves will have no reason to learn how to open the more expensive type which will leave my bicycle all the safer!  

Bike locks cost real dough because they are designed to stand up to real abuse.  

I once had a job where I routinely went around a park with a pair of huge bolt cutters, removing the bike locks (and sometimes bikes) that were abandoned there.  The cheap locks are a hellva lot easier to cut off of railings than the beefy OnGuard types.  

You do get what you pay for, here, at least in the range of $0-50.  Cheap locks are almost not worth buying.  A $50 U-lock will stand up to several tries with a pair of bolt cutters that have three-foot levers attached to the handles.