I made the easiest advent calendar this year:
1 Santa decoration from Dollar Tree
1 bag XL cotton balls
Glue of any kind
Total cost: $2.50.
All you do is take the $1 Santa and write the numbers 1 - 25 on the beard. Be sure and space them out well if you plan to use the XL cotton balls like I did. Every day you just glue a cotton ball over the date, and Voila! A super-cheap and easy, fun advent calendar for the kids. BigBoy loved it and it was cute on our pantry door. I had one like it as a kid and loved it.
I think this will have to come back next year.