I had all of these supplies lying around the house - so cost to me - $0 for a fun project that took about an hour. If I had drew- or found other pictures it would have taken me less time.
Gather your supplies. I used a canvas and 24 kraft boxes I had lying around the house. You could of course use any kind of boxes, (altoid tins etc.)
Paint your canvas.
Find 24 images, numbers, magazine cut-outs for your box lids. I designed and sized these in photoshop with mini numbers in the corners. I printed them on matte photo paper so they would peel and stick - but you could print them on regular paper and glue them just as easily.
Separate lids and boxes. I lined up my boxes on the canvas to see where to glue them. You could measure them - but I just eyed it.
Using your trusty glue gun (or any kind of glue) glue your boxes to the canvas.
While that is drying, cut peel and stick your images to the front of your boxes.
Attach the lids back to the boxes.
Admire your lovely advent calendar. Don't forget you've got to fill those boxes with little surprises.
Display it on your mantle. And its easy to keep and store for next year!