I know all about supply and demand, so please don't hit me with that.
They are super easy to grow, are in large supply, don't need to be checked for bugs (at least not yet...).
I have been informed that in the "early" years of KAJ, Rav Breuer zt"l insisted that the shul sponsor the hoshanos free of charge for those davening there.
I have also learned that as prices soared in the following years, Rav Schwab zt"l said that Hoshanos may be re-used and one need not purchase a set for each and every family member individually.
But this still begs my original question:
Why do Arovos/Hoshanas cost so much?