We live in a capitalist society where the value and price of any given commodity will be directly determined by the interrelated concepts of supply and demand in that the more readily available a particular resource it is, the less it will cost the purchaser and the converse is also true which is why gold is far more expensive than the likes of coal.
Don't limit what they can offer due to the fact that they don't have many supplies to work with. You can even consider offering them craft supplies such as glue and glitter, cotton, ribbon, and other items that can all become a part of the Spongebob coloring pages they are completing.
For the last few years kid's coloring pages printed from the Internet have become an very serious competitor to the coloring books purchased in the store. Now you don't need to pick one of the various themes to buy for your child. You can get it all. Nowadays nearly every animated cartoon character has a website with free printables or coloring pages as a bonus.