When you add in the "extras" that are required to use Green Mean mowers the costs go WAY UP. Their stated claim to ground speed is 8 to 9 MPH but with a slow BTS of 17,000 FPM (most commercial mowers are 19,000+ FPM) I can't see them hitting top speed in anything but the easiest of conditions. The longest run time is 3.5 hours under optimal conditions. I suspect a real world time is more like 2 to 2.5 hours. That means you need 4 complete battery sets to work 8 hours. A battery set consists of 4 batteries. That's 16 batteries and their batteries are expensive, their word, not mine. Then you'll need chargers to charge all of these batteries. Each battery get's it own charger. They do have a $200 charger but it takes all night so you'll need 16 of those @ $200 each. That's $3200 worth of chargers. They do have a 2 hour charger but those run $475 each and you'll need a minimum of 4 of those @$1,900 and you will have to keep switching batteries every 2 hours all night long.
I suspect you're looking at $20,000+ just to get a mower that will run 8 hours a day.