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.
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. These things take valuable space in your backpack, in your trade windows and, while they are the most natural unit of worth they're just impractical. Think of it as the one dollar bill: it's fundamental, but you wouldn't buy a car with several thousands of them.
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. This brings us to a shoddy, shifting, complicated system that makes the Imperial measurement system reek of sanity.
The following are guidelines; the relative values of these items are always shifting.
- A ref is crafted from 18 weapons (or, in terms of time, roughly two thirds of a week.)
- A key is worth somewhere between 2 and 3 refined.
- A bill's is worth around 8 keys.
- Buds are worth around upwards of 3 bill's.
- A Max's is worth around 2 buds.
- A HOUWAR is worth around 10 Max's.
Finally, there are TF2WH credits for the users of the service. A refined is 4,500 credits. And if you're a fan of the US dollar, well... tf2finance.com says earbuds are about $28, which makes a key worth about one dollar.
Q: How can keys that can only be obtained by paying $2.49 be worth less than half as much?
A: Scammers buy keys with stolen credit cards for money laundering purposes. (It's not quite as simple as that, but what were you expecting from a footnote?)
Why use these particular hats as currency? The community chose those hats as units of worth because:
- They are relatively rare (indeed, rarity is the main source of their value.§)
- They can no longer be obtained (so they're only going to go up in value.)
- The original trading system didn't let you trade more than 8 items at a time. Trading more than this was extremely risky and this is why there are so many "currency hats" that are only so much more valuable than the next.
- There's only so many items to go about anyway.
Perhaps irritatingly, this means for example that whoever happened to log in on TF2 on the Mac release week has a small fortune out of nowhere. Who preordered an entirely unrelated game has twice that fortune for it. It's arbitrary, but that's the way it is.
Now, for your question. The answer is: who knows! It depends! It changes! There are many possible sources that attempt to answer such questions:
- TF2WH.com in terms of the TF2WH credit.
- backpack.tf in terms of ref, keys and buds.
- The (in)famous TF2 spreadsheet.
- Price checking threads in all kinds of forums, sites and subreddits.
In the end, however, your item is really only worth as much as you can sell it for§. The single most reliable tool at your disposal is searching for similar trades on trading websites:
See what people are selling your item for. See what buyers are willing to buy your item for. Ask in public when in doubt and never get rushed into a trade. These are the only way to make sure you don't get ripped off. (Indeed, data from trading sites is where the fancy graph you see in tf2finance.com comes from!)
§ Rarity isn't everything. Consider this frequency distribution of effects on unusual hats: it's pretty obvious that not all effects are equally likely (and basic statistics will confirm this is no uncertain terms). Now, the "Circling TF logo" effect is pretty rare and yet it is one of the least valuable effects because people just don't like it very much. As a result, they're not willing to pay as much for it.
For example, at the time of writing, a Towering Pillar of Hats with Circling TF2 logo goes for $80, whereas the version with searing plasma goes for $500 — and yet, searing plasma is roughly as rare as Circling TF2 Logo. See also: "themed unusuals" (Blizzardy Storm is relatively common, but couple it with A Rather Festive Tree...).
So what about the valuable monstrosity of a hat that is a Bill's? Just look at it, it's a disgrace.Well, those hats are the exception because they're used as currency. People seek these hats because they're rare, not because they're dozens of times better of a Towering Pillar of Hats. If Valve released more of these items today, their value would take a pretty deep dive and much teeth would be gnashed.