Vistaprint.com is running a deal right now for 100 free postcards. If you're happy with their designs and just want to add your own text, your entire order will be about $6-7 (shipping to arrive in about 3 weeks).
If you want to upload your own image, that's an additional $2.49. (Tip from someone who learned the hard way - if you want to do more than one image, make a composite of your own images in photoshop or gimp or paint first - one uploaded composite image is $2.50 instead of $2.50 for EACH image. I corrected it - but it took a lot of extra time.)
You can leave the back blank, fill half of it and leave room for mailing addresses and a stamp, for fill it up all the way. If you do any of the above in black and white - it's free.
If you're sending it as a postcard, you're done. Check out and pay for shipping.
If you'd rather send it as a card, you can add 100 envelopes to your order for about $3.50 (you can get the fancy ones for $30, but that wouldn't be a budget tip from me, now would it?)
Our total for 100 postcards (7 photos on the front, our Christmas letter on the back) and envelopes - plus slow shipping: under $14.
These added to the 30 that I got through Kodak's Million Thanks $15 credit (I paid about $5 with overage and shipping) will completely cover our Christmas mailing list.
And if you're smarter than I am, you can do all this through Ebates and save just a little bit more with 6% cash back! I forgot about that until I was done loading the card... for the second time... and decided my time was worth it to skip the deal this time.