The muffin tin in the above picture actually rusted that day because the animals soaked for too long.
I set the muffin tin out on our deck with the intent of re-storing it somewhere besides my kitchen (like for a future mud pie kitchen) because it is pretty much un-usable now. While it was out there, Schmeese found it and started gathering rocks to put in each cup. Totally on his own with no prompting. He did that for like 15 minutes.
Muffin pans are wonderful. I can remember my mother using them with my little sister. She went to the local "teachers lab" where they had an Ellison press and she cut and laminated tiny paper hearts (maybe like an inch by an inch) and she would have my little sister sort them into the muffin tins by color. She has also used colored buttons for sorting. You could use little shapes or letters or numbers.
Cost: Pretty much free unless you wanted to go and laminate the little paper hearts