I bust mine just 3 weeks after having the car. I caught it on the side of the garage reversing out, because I was concentrating more on the aerial hitting the roof. Although the whole unit fell to pieces on the floor, and scouring the glass, the only piece that needed replacing was the glass, and Ford dealer got the glass the following day and fitted. It cost around £33, remember it is a heated unit. Pop it into your dealer and let them ascertain whether the casing can be salvaged. Good luck.
To be honest when you consider todays prices £50 for a cowling and £ 33 for the glass is quite reasonable. Several manufacturers charge over £ 100 and do not have the option to purchase components, that is if you break the glass you have to replace the whole unit.
unfortunately the Fiesta Door mirror is not compatible. They look identical but the lower black mirror base that attaches to the door has a different angle on the Fiesta to the B-Max, with the Fiesta having a more curved door top than the B-Max (which is quite flat by comparison). This would mean that even if the mounting fixings are the same etc, it would sit at a lower angle and look wrong.
I don't have details, but there is a company, (probably more than one) who will supply any glass for any car, heated or otherwise. I broke the glass on the VW Caddy I used to have. I ordered a new glass and had it fitted less than 24hrs after I broke it. Google is your friend.