I have done both CAD and 3D models of architectural plans. It is a time-consuming process that I would bill out at approximately $800 to $1000 depending on the scope of the plans.
If the client just want the plans they have printed larger there are cheaper ways of accomplishing this. You can have them scanned on a oversized scanner then print them. Also many architectural drawings are in PDF format complete with vector information making enlargement simple. Go to a printer that specializes in architectural printing they should have the equipment to take care of it for you.
Assuming the client has obtain these plans through legitimate means, go back to the architect. They will overcharge for simple copies but it is cheaper than redrawing the whole thing.