I spent this morning happily emailing with two friends in Marin who are hosting holiday parties on wine recommendations for their party needs and budgets.
So I might as well pass on to you some of the options for affordably priced party wines for your holiday (or any occasion) big volume, low price wine needs - all with 100% certified grapes and from the U.S.
I've never made mulled wine. All I know is that it's red and that people used to mostly drink sweetened or flavored wines for most of human history. Since you're going to be adding spices and changing flavor of the wine, you don't need a high quality standalone wine. I hope the wineries I am recommending won't be offended, but the recommended choices I've come up with are the most inexpensive organically grown reds I could find and three of them happen to be sulfite free as well (the first three listed):
Generally in stock at Whole Foods
• Our Daily Red, $7-9 a bottle
Generally available at Trader Joe's
Orleans Hills is another of their wines and would be a comparable buy
• Badger Mountain Pure Red - 3L box, $24 online
This works out to the equivalent of $6 a bottle. Haven't seen it in a store yet, but one can order it online for sure.
• Preston's Guadarni, $34 for a jug (equal to 4 bottles or $8.50 a bottle) - available in person from winery on Sundays only; limit one per person
A personal favorite for me (get the matching glasses there, too) but very limited in terms of availability (i.e. you must purchase at the winery only, only one per person, and only available on Sundays). Nevertheless, if you plan (and shop) ahead, a steal.
POSTSCRIPT - Barra's also have a very big sale. So I'd definitely add their sales wines. (See preceding