It's always tricky to quote for removing tiles. If they have been tiled on plasterboard you're going to make a mess of it and probably will need to replace it. If it's plaster on render on the exterior wall and plaster on laths internally, again, that's a pig to correct and you'll pull of the tiles carefully and then patch up and re-tile. If it's a bit of a mix then again it's a nightmare to quote for.
What I would do is say to the customer you want X a day, should take you X days/hours, you'll quote them X much but be warned it could be a bit more. Then keep them posted with how it's going and they wont feel ripped off if you need to stick on another £50-80 whatever on the bill.