Smith
I hate to be the one to break this to you but Beauvais is not in Paris, it's more than 100 kms north of the city, calling Beauvais Paris is like saying Reims is also in Paris, it's a different city.You'll spend 2.5hrs each way getting to and from Paris.
Paris has two major airports - Orly and Roissy, you can get to the centre of the city quite quickly from both.
If you're just intending to go and have lunch you'd be better off staying in Beauvais. If you do finally make it to Paris expect to pay more than 2p for lunch, quality food in Paris costs money, just like quality airlines. Though you can probably get lunch in any Paris Macdonalds for less than €15....