Guess work for us all, of course.
Im unconvinced Gray would actually be cheaper (at all/by much). Leicester have no need to sell, and given they have him under contract etc would demand a fair bit I am sure.
Zaha? Recently signed a new deal I believe, could mean one of two things. 1) Ridiculous price due to long contract or 2) Buy-out clause inserted/assurance of sale. Could go either way.
As to the players, while I appreciate Gray is younger, Zaha is a beast physically, more experienced and mature and IMO set to step up to bigger things - at this moment in time he would be my preference.
That said, Gray is an interesting player whom Id be happy to see come in.