New grinders are about 25-30 thousand, depending brand, size, and options.  Used grinders are cheap, depending on age and condition.  If it's something that will be used several times per day 365 days per year, it needs to be reliable.  If memory serves correctly, electric rollers were expensive and costly to repair if anything got into them.  It seems to me they were once an option on PTO grinders instead of hammer mills, but I've never seen one.

It used to be almost every farm in the neighborhood had livestock and a PTO grinder-mixer.  Not too many have livestock now.  Many of those that do are cattle yards with little need for that type of grinder or hog farms that have the feed delivered complete.  The market for PTO grinders is nothing like what it used to be, but the concept still puts the functions of grinding, mixing, and transport into one neat little package with few strings attached.  Four tons is bigger than they used to be, but I would have thought someone would have been making something about twice that size these days (not that I really need one that big).  Perhaps the market is not really there for something like that.  Operations with that many hogs probably have multiple locations and would want a truck to transport the feed longer distances.