how much would it be to fix a cracked iMac screen ?

Hi,

Apple will have to determine that for you > https://support.apple.com/mac/repair/service

And you can make an appointment at an Apple Store Genius Bar if that's convenient for you.

"...Available for both the 21.5" and 27" iMac, The Macframe will work on all iMacs with an edge to edge removable glass screen (Late 2009 up to Mid 2012*). Because of the design of the Macframe, it is not available for any earlier iMac versions or late 2012 and newer "Thin Screen" iMacs..."

http://www.macframes.com/

if you happen to have an older iMac 2009-2011, and want to get an anti-glare matte replacement glass...and the damage is only to the external glass, not the monitor itself, this might work