I personally think it's a brilliant tournament, which is why 2009 will be my 6th year in succession attending it! At least the 2 courts are very close to each other, unlike in Antwerp for example, where they were about 400-500 metres walk!
You'll struggle to get much below 70 Euros per night close to the tennis (you can pay around that price on a couple of decent small hotels which I've stayed at before on Rue Boileau, 5 minutes walk from the tennis called Hotel Boileau which has a website at http://www.hotel-boileau.com/en/welcome.html
and Hotel Exelmans). I'm paying quite a bit more than that next year at a Mercure the other side of Porte Saint Cloud from the tennis.
If you look for the cheap Etap brand of hotels at http://www.accorhotels.com
you should get it lower priced than that, but you'll have to get local trains to reach the tennis, as you'd inevitably be staying on the outskirts of Paris. I guess another option is youth hostels.
Just looking at www.expedia.co.uk
, they have a few around the £45 range that are probably around 45 minutes from the tennis on the metro.
Stefi, about seats, yes you can always get tickets, but the best seats ("Premiere") are sometimes sold out for the Tuesday or Wednesday before the opening weekend. Although they're much expensive, I think the premiere seats are more comfortable too, especially if it's a long evening session!