This is actually variable but it depends on the following question:
Do I want to get steam funds back or do I want to buy stuff with it from trading?

If you want steam funds back go to the SCM and search "key" then sort in descending order by price. Here is a link to that page. The first case key on that page is what you want to buy. Currently that is the crimson cache key with a price of $2.73 (or $2.72 to quick buyers). Now why do you want to buy this over the normal key? Well it's pretty obvious if you look at the normal key's price: $2.16 at time of writing. That's 48 cents lost after fees for buying something that costs the same price in the in game store! And that's on top of the loss you're already taking from fees (17 cents if you aren't taxed).

If you want to buy stuff from trading the easiest way is to check what scrap.tf pays for keys which you can find here[scrap.tf]. Ideally you would search every backpack.tf classifieds but that is very time consuming and with scrap's pricelist all you have to do is ctrl+f "cosmetic key" and "war paint key". Even if they have full stock it's still likely the most expensive key they have will be the most expensive key you can get when selling to backpack.tf bots, but honestly it doesn't matter too much since the prices will fluctuate in the week it takes for the key to be tradable. Currently doing this means you get an extra 19+ ref for free over buying a normal key.

Finally, you only have to buy ONE item from the mann co store. Any other item you want is probably cheaper on the steam community market, such as backpack expanders, which are currently 86 cents vs the normal 99 and if you are taxed in the mann co store you will also avoid more of those fees by purchasing on the market.