6 inch diameter is a massive hole I got advisories on holes that were only a few inches across. I don't know how much it will cost to get welded I'm afraid as I welded mine up myself. At a guess, you probably won't get much change from £100 and it will also probably depend on how much of the interior trim you remove and refit yourself.
Your best bet is to take the car to a few back street garages and get some quotes.
My advice would be to get the rot cut out completely. If you just weld a plate over the holes (a bodge IMO) then the tin worm will just continue eating the metal away under the plate and will also start attacking the new plate so you will eventually just end up back where you started if you keep the car long enough.
To give you an idea, the holes in the following pics were an MOT advisory when they were about half the size they are in the pics. It's when they got that big that I decided I needed to do something about them before I was handed an MOT failure sheet...