The $850/month includes the following that you would have to pay if you bought a petrol MINI yourself:
(a) The cost of the petrol (offset in this case by the minimal cost of recharging the MINI E)
(b) The cost of a (possible) finance repayment.
(c) The cost of depreciation.
(d) The cost of general wear and tear (tyres etc.) that you would incur on your own car, but won't incur on this one as you just hand it back at the end of the year.
Add up the cost of (a) to (d) above, add in a bit for the experience of driving the car, and put a value on the exclusivity/novelty of an electric MINI, and you'll probably come to $850, so it could work out quite well...having said all of that though, I'd still prefer a petrol car than a battery powered one!