We are looking for an architect who can make somewhat more specialized proposals regarding house construction floor plans. Does that immediately make it much more expensive?
Expensive is always relative, isn't it? He certainly won't work for free... It surely depends on the effort involved. As Hans already writes, he will charge an hourly rate. Maybe you can also agree on a fixed price...
The architect will certainly not be more expensive than usual. But I would also agree on a flat rate if possible. Architects are used to extraordinary challenges anyway. But it is definitely better to discuss everything with the architects.