We sample a lot of champagnes/sparkling whites in our house. Here are my suggestions, based on significant consumption of each over the years...
There are three good choices in the $12-18 range that are easy to find. All three are head and shoulders above Korbel, Conte Negro, and anything Asti, in my opinion. In rank order, they are:
1. Mumm Napa Brut Prestige (NOT Cuvee)
2. Piper Sonoma (yellow-gold label)
3. Chandon Brut
For about $20-25, there is Roederer Estate Brut (also the maker of Cristal) that is excellent value.
In the $35-$45 range there is:
1. Perrier-Jouet (sweetest)
2. Moet & Chandon White Star (mid-sweet)
3. Piper-Heidsieck Red Label (my fav - nice and dry)
Lots of really good choices in the $50+ dollar range, but unless you drink a lot of champagne – or are trying to impress a dancer in the Champagne Room – not worth the extra cash IMHO.
Have fun!
L4L