
A Hat of Undeniable Wealth And Respect is worth around 56 buds. A Max's is worth around 19 keys or 12.5 buds. Buds are worth around 1.5 keys, around a bill's.
A key is worth somewhere around 35 refined. A ref is crafted from 18 weapons (or, in terms of time, roughly two thirds of a week.). The following are guidelines the relative values of these items are always shifting. This brings us to a shoddy, shifting, complicated system that makes the Imperial measurement system look sane by comparison. So, we need units of worth that are worth more than a refined metal, making do with what items are available in the game today. (They've also lost half of their value in the last year since this was a thing that happened, as if it was hard to have multiple accounts on TF2 before.) Think of it as the one dollar bill: it's fundamental, but you wouldn't buy a car with several thousands of them.
These things take valuable space in your backpack, in your trade windows and, while they are were the most natural unit of worth they're just impractical. The idea is that while accumulating refined metal is good and easy to make and doesn't require interactions with other human beings, it's not a practical unit of trading "worth" in transaction of items "worth" several hundreds times a refined metal. Well, define "worth." In order for you to understand what these items are "worth" you need to understand the context that gives "worth" to these items: the TF2 trading economy.