If you can wait, I'd suggest holding out for two weeks. The summer sale should start around then and you'll be able to buy a lot of games for $30.
Portal 2 has been on sale for $7.50 a few times. I picked it up and found it worth that, but nothing more, I really didn't care for the game much honestly. The best part was the writing, the puzzles were easy and even though it isn't a long game, it started to feel long near the end I was just wanting to finish it.