URGENT!!! alloy wheel lock nut cracked?

How do i remove a cracked alloy wheel lock nut, when i turned the key in the nut it cracked down the side a bit an now the key doesnt fit, i dont want to do damage to the alloy the nuts are 20mm deep with 1 side in accessable as it sits in with the alloy, im looking for a way to do it with very little cost involved, thanks