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I've lost my v-card a few times by this
Hear Ye, All Ye Hungover Masses, Happy Sunday!  Screw Funday...it's Bubbly Day! 
"What's the best, least expensive champagne for mimosas?" is a question I've had in the "Nickel for every time I..." category.  Bill and Melinda would be my pool children.  

Dear Mary,
You aren't asking about Champagne.  If you were, you wouldn't have walked up to me in a hoodie and smoke-saturated jeans, loose from the night before, with your girlfriend's lipstick fresh on top of your 1:00 shadow (our "example" character is from east of New Foundland).  You're looking for "sparkling wine".  "Champagne" is a sparkling wine from a place called Champagne, a locality 100 mi. west of Paris, France.  Do you know where France is?  Of course you do[n't].  It requires 18 months in an oak barrel to age and has yeast fermenting within to create a rich character many refer to as 'toast', 'vanilla', etc.  Your toast is what you are when returning home to your girlfriend after an afternoon with moi.

What you need: For the an excellent mimosa, do not use Champagne.  It begins at $35 and doesn't make sense to cover its nuances in orange juice.  Piper-Sonoma is a sparkling wine made in California by the same authority that produces Piper-Heidsieck in Champagne, France.  Their Champagne happens to be one of the best values in the region.  Their sparkling wine doesn't differ by much and uses the same method, minus the terroir of Champagne.  Juice?  Vodka is a fave but my friends use either orange or lemon.
Piper-Sonoma Brut, Sonoma County, California
avg. retail $14