Deals on SC2 are going to be very rare, same with all of Blizzard's games.
Still, for what you're looking for it's easily worth the $50. The game is very similar to the original as it is very straight forward and easy to understand. Unlike the original however, SC2 is also very easy to become good at, while the original was much more challenging (and arguably much more intense as well).
Not to say SC2 isn't fun, because it's very enjoyable. It's just catered to a wider audience now and I think if that's what you're looking for then go for it. It's very competitive, easy to learn, has a pretty fun campaign, and then there's always user created maps that are insanely fun to play.
The game is a hobby in and of itself, the current price is completely justified.