-=::) Dante (::=-
On the one hand money in Power Grid is a trackable resource so why bother hiding it?
Because the designer himself has supposedly expressed in a thread here (that I've only ever heard referenced but not seen myself) that the money is intended to be hidden rather than open, as open money leads to much more AP especially in the final turns.
On the other hand, if your group doesn't track these things, the plastic coins can be stored in a pocket with much less chance damage to the components than putting paper money in your pocket
Exactly what the countless groups already do who use poker chips. Just pocket a couple of higher value chips, problem solved.