Bought Terminator Resistance yesterday for full price and now it's on sale!
Was anybody ever able to get a discount afterwards in this sort of situation?

No and you shouldn't be able to as it's your own choice to buy that day and not wait to another day where there are discounts. It sucks but that's life :)

Sometimes forum users have managed to get it, if they bought the game just before a big discount. You can always contact Support and be very polite about it. The worst that could happen is that your request gets denied (i.e. what you already have).

Always wait until the end of a sale before buying anything at full price... if it's any consolation to you, I also learned this lesson the "hard way" :).

Try support, helps if you didn't downloaded it yet

Well if you got it yesterday just contact support and they might be able to give you the difference in wallet funds..At least that's what they did with other people here in the forums.But next time just be more careful :)

Like bela555 said, if you haven't downloaded it yet, try support. Just tell them you'd be fine with getting wallet credit and you'd rebuy it right away and spend the remaining 10 bucks on some other game. If you're a regular customer and never refunded anything before, you might try your luck even if you've downloaded it already. As long as they don't get the impression that you're trying to game the system, they may be accomodating. Good luck!