Least expensive option Beauvais Airport to Bastile, Paris

Here is the Beauvais Tille aeroport site:
http://www.aeroportbeauvais.com/index.php?lang=eng
Click the Informations button and then the Connections button to see your alternatives.
The shuttle bus into Paris runs to Porte Maillot which gives access to the Paris Metro for the Bastille in the city centre. Metro tickets are 1,70€, a little cheaper when bought in a carnet (package) of 10 tickets. The subway ride on the M1 line takes about a half hour.
The TER trains from SNCF run from Beauvais itself into Gare du Nord but the train stop is 5.5 km. from the airport by bus shuttle or taxi. A train ticket into Paris costs 14€ and the journey takes 1hour20, then M5 subway to Bastille taking about 17 minutes.
So you can do the math to compare the various totals for a group of five. Clearly a taxi large enough for your group (with luggage) will be the most convenient and the most expensive. That's the payoff for using low-cost plane routes: Save money, at the price of time and ground transportation to your final destination.

In the end, we took the Ryan air bus, then two metros because the taxis were on strike. It was a pain and certainly not worth the small savings. I would not recommend taking Ryan Air into Beauvais unless you need to go to Beauvais.

I once read that the only people who say that Beauvais Airport is in Paris are Ryanair employees. It is a royal pain to get to Paris from there.

it isn't just that it is so far outside of Paris, it is also that there is no decent public transport -- CDG is well outside Paris too but for 9.75 you can take the train in and don't worry about having to miss one as they run every few minutes.

Easyjet and Vueling are two budget airlines that usually fly into the regular city airports including Orly and CDG in Paris.