Now, I love Champagne, but we're talking about a specific situation here where money counts and breathtaking quality does not. We want delicious and delicate, cake-worthy and crowd-pleasing, and for that we can look outside of France. Consider Cava (Spain), Prosecco (Italy), or shut down the search and just order cases from Gruet, in New Mexico. Their phenomenal flagship Brut goes for around $15 a magnum, and will please everyone from Aunt Edna to your cousin the wine snob.
For those working with a caterer, check during the introductory process whether they'll source the things you want, or if they charge corkage (and whether it's exorbitant) if you opt to select and purchase your sparkling wine on your own.