I have a regular easy Master Lock 140 in my naughty bucket. It has hard biting but that shouldn't make it as hard as it is. I've spent an hour working on the stupid 140 and can't get it, but then I can pick most other locks at the yellow belt level and a few at the orange belt level in a few seconds. As I've gotten better and opened harder and harder locks my master 140 just pisses me off even more. Once in a while I just comb pick bypass it to make myself feel better.
My other naughty bucket lock is an Abus 80TI/50 which is a significantly harder lock. I can spend an hour on that lock setting pins while repeatedly dropping other pins. It seems like every time one pin sets in that lock I drop two more.
The last lock I own and haven't been able to pick is this Stanley lock. I'm not sure I'd classify it as naughty bucket yet. It's just a hard pick and I think it's beyond my current ability. I actually bought that lock to secure something years ago, and I'm glad it's hard to pick because I feel like I made a good purchase for security.
This is a little above your price range, but still cheap. I use this Stanley lock for actually securing things. It's not listed in the belt rankings. It's a hard pick for a novice with serrated pins probably close in difficulty to the American Lock 1100. The real reason I use that lock is for the physical security. It has really good anti-drill measures in it including hardened pins and ball bearings protecting the pin stack. It's significantly beefier than the American Lock 1100.
Lockpicking Lawyer has a pick and gut video for this lock if you're interested in what's inside of it. He seems to be a fan of the lock and ranks it pretty high on his list especially for the price point.
On a semi-unrelated note: how do you get a lock added to the belt rankings? The belt ranking doesn't include any Stanley locks, and of the locks I own this one is probably the most difficult pick. They also have the higher security Stanley CD8820 with serrated and spool pins. Both are guttable and would probably be nice additions to the Blue Belt list.
Alright, I don't know what conditions or environments the lock is gonna be in, but here's my two suggestions. The American is a bit harder to pick, but slightly less physically strong as it lacks the shackle guard. It is however made out of solid steel.
I really like the Stanley locks personally. They are rekeyable, bit an arm and a leg, and really nice. I own three of them. (2 60mm padlocks and a 50mm with shackle guard). The 50mm is under 16.00 even. Here are the three I have purchased that I am really happy with:
That being said, if it is a storage unit that is enclosed, I would have no problem using a normal Chateaux discus lock. I use the Chateaux C970 discus lock, but now that I have found the 50mm stanley with shackle guard I am going to start using them since they are actually a little cheaper than the Chateaux and gives me the option to key it any way I like (thanks to the removable cylinder).
HTH