How I do Disneyland Paris:
-2 Adults usually
-5 days, Monday to Friday in the off season, usually Sep/Oct/Nov
-Get two 2 park/5 day park hopper passes (highly recommend the hopper option since the Studio park is pretty small currently)
-Eurostar via London to Marne La-Vallee (Disneyland) directly both ways, though flying may be a cheaper option depending where you live.
-Stay at Disney's Sequoia Lodge Standard room (a moderate hotel) or if you're on a budget then there's nothing wrong with Disney's Santa Fe or Cheyenne hotels which are a little cheaper since they are value hotels meaning not as nice and further away.
-We never eat in the parks, the food is mega over priced and doesn't taste that great so we eat in the Disney Village restaurants, they are a little more reasonable. The McDonald's there is a great option if you are on a budget as well as the convenience stores out in Eurostar/RER Train station. Spend around 100 Euros over the five days since we're cheap and go for the cheapest options possible.
-Souvenirs (like at any theme park) are expensive though there are a few bargains here and there. We usually buy pins, key chains and magnets which sets us back maybe 100 Euros for the whole trip, we don't spend much on this but this varies widely from person by person.
-Ride wise we do everything, if you are two adults and don't mind coasters and drop tower rides you will be fine.
-If you want a day in Paris city then just walk to the train station outside Disney Village and get tickets for the RER A line and hop on the train, 40 minute ride each way.
So hotel, park tickets and Eurostar for us usually comes to a total of about £850, souvenirs I budget a little over £100, and food is also usually £100, so overall on the cheap our visit comes to £1,200 overall, so you're looking to spend about £600pp on this trip if you do as I do.
I recommend studying the following websites to get a good idea of the place:
http://www.disneylandparis.co.uk/
http://www.dlpguide.com/
Map of the resort: