Cheap locks offer more of a
Cheap locks offer more of a visual deterrent than actual security. I’ve seen £20 cables broken by hand, the best advice is always to buy the best lock you can afford, with the hightest security rating (I always look out for locks with the German or Dutch ratings cos they are really tough) and then to lock you’re bike properly in as secure a place as you can find. Brands to look out for: Kryptonite, and Abus at the top of the tree price wise, Magnum are pretty good too.
If you’re on a budget a cheaper d-lock will give more protection than a cheap cable. I don’t rate cables that much tbh – unless they are top of the range jobs, then they become so heavy that they start to lose their advantage over a d-lock.
The thing to remember is that if you give a thief the time and opportunity any lock can be broken. A cheap lock properly used will give you more protection than an expensive one badly used.