Pheace: I really think this discussion would be helped immensely by you telling us which country you were in, yet you seem to be avoiding giving that information on several occasions now. If you have some reason to not want to that's fine of course, but just putting it out there that it *is* relevant when you buy something in a country different from the US.
Sorry, I thought I did lol. I was in Turkey at the time of purchase... I am not there anymore, but in Greece and I still have the same issue (but with the base prices, now that the sale is over). I suspect it is as you mentioned a couple of posts above, and that the browser (but this would not explain the client's behavior) was set to the US store.
abolat: With regards to how, I have no idea to be honest. Haven't had a problem since I left the US though...
bansama: Quite frankly, I'd be very careful with how you handle this. If the game was $12.50 in your current region and you keep pressing this and Valve find you were purchasing overseas with a billing address that doesn't match your current region, they could very well disable your account permanently.
They don't take too kindly to people trying to circumnavigate regional pricing/restrictions when purchasing themselves directly (i.e., not gifting/trading).
And as I say, the fact you were even able to make the purchase via the client (I gather?) is rather strange as you tend to get a generic error in such cases that prevents the purchase. When overseas Steam support will generally instruct people to purchase from the website using the ?cc=XX method (and I assume after removing some regional block on their end) but purchasing through the client is generally not possible as the region cannot be changed (without the use of proxies/VPNs).
there was no circumvention of regional pricing... at least none to my benefit (have always been billed more than I would have in the US throughout my travels).
Just an edit: You might be right pheace and bansama as I bought this though the browser. The client does not let me enter my billing address properly, so I cannot use it for purchases.