I've been doing some (ok, a lot) of research as to the pricing of holiday cards. I'm personally geared toward the nice 4''x8'' that you mail in envelopes.
Know what I've found? There is a crap ton of variation in pricing.
Here's the best deal I've found so far:
50 prints for $11.99 (with envelopes included) is around $0.25/card. Not $1.50/card like I've seen some crazy companies charging.
I was a little wary of the $20 additional charge for envelopes when the envelopes are noted as included here when I was doing the instant quote service, then I realized you just have to go back and change the envelope selection to the free choice (tricky tricky).
The only thing that sucks is the shipping and processing fee and lead-time.
Shipping and Processing Costs:
Rush - $25.19 - 3 Business Days
Priority - $13.15 - 7 Days
Standard - $9.15 - 14 Days
Slow - $5.45 - 21 Days
In all honesty, they seem to be the cheapest for any type of printing (business cards, etc.) I'm ever doing anyway.
I'll be sure to update as this search goes on and post some results later down this holiday road.
PS - Their customer service is very helpful by phone, too.
PS#2 - Walgreens is also a good choice at $9.99 for sets of 20 that you can pick up at the store (ie: no shipping costs), about $0.50/each. I don't think you can create your own design though... you may be restricted to their templates. I'm getting really creative (thanks to iStockPhoto.com and Adobe Photoshop CS3) and making our own design.
PS#3 - Vista Print is offering "Get 50% OFF Holiday Cards offer ends 11/16/2008" currently... now that's a REALLY good deal.