How much would the 12 Days of Christmas cost to buy? PNC has been following the costs for 28 years and provides a great interactive game for students to explore the costs including percent of increase and decrease from 2010. This activity may be used with or without the online interactive game. However, if you choose to not use it, you will need to provide the information. There are four parts to the activity. This is a fun holiday activity to review percents and estimation.