You still have to invest the mental stress just like evocation.
So, if your conviction is 7 after foci, you could have a 7-shift ward. It would cost you 1 mental stress.
If you wanted a 14 shift ward, you'd need to spend a 4-box of stress and a moderate consequence (or 2 minor mental consequences, if you have'em.)
THEN, you still have to try to control it.
Can you get your sponsor to ante the power so that you don't have to take some of the mental stress?
Maybe yes? depends on your GM. I'd say 1 debt for every 2points of stress you want the sponsor to soak.
Can you get the sponsor to suck up the backlash when you fail to control 14 shifts of power?
Yes? How much backlash/debt is up to the GM/table.
The GM might agree that, for this one spell (assuming it's fits the agenda or the sponsor is trying to suck you in) the sponsor will take it all. Maybe 1 debt for each 'invocation' until you can control it (1debt:2shifts)
Depends on the story and the situation and the GM and the table.