Accommodation isn't that cheap, but if you shop around you can find reasonably priced hotels or apartments. I'm heading back to the same hotel I used this year which was the Ibis Montmartre, which is costing me about £65 per night for a room. It's close to a few Metro stations and only about a twenty minute metro ride to the start area.
Paris is an expensive city, but you can do it reasonably cheaply if you want. I was over for four nights last year and didn't spend that much on food. Other than the pre-race meet-up I eat in a couple of Italian restaurants near the hotel that were what I'd say was average high street UK restaurant prices.
ATM is the best person to give the Paris low down and Radar Sal put together a supporters guide last year. From what I heard of that Paris seems to be better than most marathons for family and friends wanting to catch their loved one running the race as it's easy to get to various sections of the course.
Just tell your good lady about the French firemen on the course. That seems to get a high rating approval from the ladies of this thread!
Edited: 13/12/2012 at 14:18