Domaine Ste. Michelle
I found this vegan champagne online for between $8.99 and $12.99, so this seem to be the most affordable option.
This one usually runs about $14.00 per bottle. The Brut and Extra Dry by Korbel are not vegan.
Prices vary greatly (depending on type and vintage); the least expensive bottle I found was on Amazon for $175.00.
The least expensive option I found when searching online was NV Veuve Clicquot Yellow Label for $60.00 on Amazon.
Moet and Chandon
The bottles you can easily find at liquor stores and grocery stores run around $55.00 to $65.00.
I could not find a price or retailer in the US for this one, but folks in Europe have this vegan champagne option (and I'm sure with more research, you could find it in the US as well).
Read more about Why Most Wines Aren't Vegan, and How to Find One that Is.
Find out if your fave beer or wine is vegan at Barnivore.com.