Segura Viudas Brut Reserva "fermented in the bottle" and available in my part of the world as low as $6 a bottle. We served a lot of this at a daughter's wedding in Canada (where it was a bit pricier but still attainable) and even my wine-snob restaurateur brother-in-law grudgingly pronounced it as acceptable.

As others have noted, it is not champagne, but I still like it.