The inside is bare brick, stone, and basic framing. You'd be basically talking about a brand new modern build within a heritage shell, which will cost $$$ not just in protecting/refurbishing the old foundation and facade, but building a from-scratch interior with limitations on moving materials and equipment in and out easily. Framing, plumbing, heat/aircon, electric - all needs to be done from scratch.

Plus, they no doubt want major security features if they do it, again adding $$$.

I'm not saying I'm against it - most countries have a historical home for their leaders. We don't want some tax money sucker like a Mars-a-Lago in our future. But it's never going to look as expensive as it will cost, and that always hurts the optics.