Without getting into too much detail about what I purchase for my company, I would say approvals usually depend on what's being ordered rather than dollar amount. There are some items that are a huge expense, but because they are regularly used equipment and supplies that the company depends on, I order them without additional approval. The company basically expects those charges to come through.
The things that I do get approval for are unusual items, annual maintenance & support renewals (Do we need them again? Do we need to make adjustments this time?), and items that involve a technical or process change from what we typically get or use.