I haven't stayed there but ETAP at Marne la Vallée Val d'Europe seems pretty cheap (starting at below EUR50), and the place is VERY easily accessible by public transport (just about 50 metres from the train station) and quite convenient to Paris city centre.

http://www.etaphotel.com/gb/hotel-50...al/index.shtml

I noticed the place during my shopping expedition to La Vallee Village and thought that it might be a convenient pad for future shopping expedition where I'd spend the EUR200 saved in one night's accommodation on more shopping