I saw something once about prisons running metal detector sweep of the yard. I remember seeing lots of shanks but dont recall them digging up cash, although I'm sure its probably there somewhere. Drugs, weapons, phones, cash, all of it gets smuggled in, so its hiding in there somewhere.
It might depend on the facility, but we seldom saw metallic shanks at ours. When we did, they were nearly always for use inside the unit, not the yard. Those typically made from toothbrushes (only a few inches long, specifically to preclude their use as weapons) with the blade from a disposable razor grafted onto the end. Usually though, shanks were made from wood or Plexiglass. The inmate would grind a point or an edge onto it using the floor of his cell, then transport it out to the yard for hiding. They'd be hidden by driving the point into the ground and stepping on the back. Sadly, those are almost impossible to find unless they're used. The reason for this is precisely because they can't be found with a metal detector, they're ready for use at a later date (and generally, the yard is where a weapon will be needed), and if found, it can't really be traced to a specific inmate since it's in a common area, so nobody gets in trouble for it.
Cash was not allowed inside the fence, not even for staff. Inmates could purchase commissary officially by having money on their account, which is placed there by friends and family through DoC. On the black market, goods and services are purchased with (surprise!) goods and services. A common form of "currency" were "pre-Franked" (postage paid in advance) envelopes, since they were useful to most inmates and were not contraband, meaning that they could not easily be confiscated. I'm not sure what good cash would be to an inmate, since if we found it, it would be an immediate trip to the IMU and possibly a downgrade to their classification.
I never found any cash though, not even once. Not in cells, not in yards, never. I found pretty much anything else that you might imagine, but not so much as a dime.
As for sweeping the yards, it was a miserable way to spend the night. I only remember digging up bits of wire from the fence, and only a few per night if that. Based on their condition, they'd probably been there since the fence had been installed. They were always too small and rusty to be useful as weapons.
Prisons differ, but unless it's a very old one, I wouldn't bother. You know the joke about detecting on old homesteads, right? If someone doesn't have a pot to pee in, don't expect to find many pots on their property. Inmates don't own much.
