On sale, you can generally find Piper Sonoma for $11-12, and at that price, I think it's much the best sparkler out there - especially if you are trying to get as close as possible to the taste and character of REAL Champagne. Domain Chandon is comparable, but typically a couple bucks more a bottle. I personally slightly prefer the Piper to the Chandon, and the fact that it's usually a couple bucks less is a bonus.

For a few bucks less, Codorniu Brut, from Spain is pretty good, but I personally would pay ~$3 more for the Piper Sonoma.

If you don't mind something a bit different, and typically less dry and more fruity, there are lots of Proseccos from Italy available for around that price - although I'm not up on which ones are best.