Well, the OED (which trumps Chambers cuz it's bigger!) only equates lever=prise (not prize or price). Of course since I don't mail these in my opinion should be discounted appropriately.
Took a little while to accustom myself to Mr Magoo's clue-writing style. As to 14 AC, I've gone with a 'Z', as this is the US usage/spelling meaning leverage. In British usage the 'S' form has only one meaning i.e. leverage as far as I can see. I don't have Chambers, so I may be mistaken, and would be happy to be corrected before I post my solution.
I have gone with prise. Chambers has prise meaning leverage and then to confuse things it has "prise 2 see price, and prize1,2,4." So there must be an argument that all three variants apply.
Unfortunately, 'prise' meaning to value, as in 'prised possession', is an obsolete spelling, and should properly be indicated as such in a clue like this. 'Prize' meaning to lever however is current usage, and is surely what Mr M intended here.