As Nocturnal said, they are not hard to find. I just looked on Gunbroker and there are 11 for sale there, and another 4 on E-bay.
If you want one, there are plenty out there. If you want to save some money, and have patience, you are going to have to wait for the next Benelliparts pre-order to open up.
Price wise, the market dictates that. If you want ammo, gotta pay today’s price. Want to hold out for the 2 years ago price, you ain’t gonna have any ammo, maybe ever. Want to buy some Amazon stock at the price from 3 years ago? Ain’t gonna happen. So, if you want a C-stock the current market is $850-$1000, or $500 with patience from Benelliparts. Is your patience worth the $350-$500 savings, only you can answer that.
From what I gather (and I may be wrong on this) Benelli isn’t really shipping these to the US currently. So you are looking for something no one can just go buy. Benelliparts has a connection somewhere and he gets them in spurts, along with other OEM Benelli parts not found anywhere else. Not sure I have seen the OEM muzzle brake anywhere other than from him.
If I was building another M4, and wanted a C-stock on it, I’d buy a somewhat readily available 1014 to begin with and buy a 3 position recoil tube to install on it.