Thanks for the great ideas. Here is what I did:
I made a multi page card and let my 3 year old color it. For each of the first three pages I drew a simple picture of items we use a lot and then listed how much it costs. My three year old loves milk, so we go through a lot of it. So on the first page I drew a little gallon of milk and wrote, "gallon of milk... $1.98". The other two pages had other items we use a lot and their cost. On the last page I drew a little girl to represent my three year old and a little baby next to her and wrote, "finding out that your daughter is going to be a big sister.... Priceless".
As soon as my husband got home from work, my daughter insisted that he read the card she colored. So he started reading it out loud to her, thinking it was just some cute little project we did. When he read the last page, it took him a few seconds to get what he had just read. He was sooo excited and he thought the card was great. He thinks we should send similar cards to our families.