First of all, I want to confirm that this "bug" still exists and wasn't changed in a patch: When you can begin construction of a district, you should do so as quickly as possible to lock in the production cost. Then you can switch back to building whatever it was you were building before, and resume work on the district at your convenience. This will stop the district cost from escalating as you advance through the tech eras. But one drawback to this technique is that you cover up potential strategic resources from being unearthed after you research the appropriate tech. This is especially true in a capital city since they are given a high bias for resources. Is it worth the risk of covering up new strategic resources? I think locking in is good in the early game because you don't really need to worry about covering up iron; you rarely want to build districts on hills, but you could potentially be depriving yourself of niter, oil, aluminum, uranium, and artifacts later in the game.