Would require lua if not dll. I'm not sure where the real 'inquisitor-cost' mechanics are buried -- one of those over-complicated Firaxis-provided UI Lua files, or somewhere in dll.
In any event there's nothing in Buildings-XML Scheme to adjust the cost of a specific unit. It is all hammers-required modifications, and doesn't apply to Missionaries, Inquisitors, and GPeople bought with faith.
You could do a 'backdoor' method with Machiavelli24's SerialEventUnitCreatedGood system, to detect when an Inquisitor is created, and then in the city where purchased, if the building exists, give back the 25% faith-cost to the player. It could be done in lua (which you'd have to do using Machiavelli's system anyway), but it might be a bit complicated to get the current 'faith-purchase-price' of the Inquisitor in order to know how much faith to give back to the player.
There may be a more direct and elegant method via lua but I can't think what it would be at the moment. But then, I haven't tried much cityview.lua modding, and I think that file is like other UI files in that there can be only one active at a time.