Keys are 10 Ref now, RIP.
Keys are worth an infinite amount of refined, technically. You can get ref for absolutely free and keys will always be $2.50.
But if we were to price things and put values on them, we should take a look at the shop prices. The shop puts cosmetics to a higher price than keys, which makes sense. Why? Because one is a tool and the other is something you can wear and show off. If Valve made the hats on the shop craftable, we could have an easier time handling the economy, because hats would become 4 refined, or more if it's a higher shop value, and keys would be 2 refined.
I still find it bs that the shop things that are uncraftable are usually less than a key. If that was so, what's the point in buying hats? If you ask me, making a hat lower in price for being "dirty" and "not legit because it was purchased" makes no sense. If it's because it's restricted, fine. See the fix above and keep "dirty" hat prices the same.