Locks go very far back. Certainly to ancient Mesopotamia, where there were wooden ones. They locked doors, and likely they evolved from various kinds of simple latches, where bar could be raised by turning a simple key that was a rod with a cam on the end. Metal padlocks seem to have come in a little later, as could be expected, but certainly Roman ones survive.

Yes, locks were expensive to make, before there were factories to make them. But things in general were expensive, because there were no factories to make them, either. Without lots of powered appliances to wash and clean and cook and bring in water and provide light, there were also a lot of servants in any sizeable house, So, lots of things tended to be locked up that we wouldn't bother with securing now. Lockable cupboards for bread have survived from the medieval period, as well as locking linen closets and chests for storing clothes and blankets and tapestries. The mark of honor of the housekeeper in a medieval great house was an enormous bunch of keys, hanging from her belt, giving her the power to administer the household goods, making sure the servants didn't sell a few towels, or the cook didn't peddle a loaf of bread out the back door. That belt-worn key-chain, even when holding other objects, is still called a chatelaine.

Some of these locks could be rather simple, just protection against causal theft- preventing a servant from quickly grabbing an object and stuffing it in his shirt, But locking chests for real valuables could be quite elaborate , and later locksmiths of the 17th and 18th centuries created some truly elaborate locks , including "thief-taker" locks for exterior doors that would , if tampered with, send two jaws out to grab the thief's hand, or at least guard the lock from further meddling.

Pitt-Rivers: On the Development and Distribution of Primitive Locks

Musée le Secq des Tournelles: Thief-taker lock