Proyo Yolsters were one of the coolest items to have back in the day.  Made of good plastic, these held either the regular shape or butterfly shape yoyo with a very good, tight grip and an integrated  belt clip.  While these were made by the thousands for the school program, not many exist today.
 
This is for the Gen 2 Black Yolster.
 
•  Rare meter  • • • • • 5/5
•  Condition meter  • • • • • 5/5
•  Estimated Value $10 - 15