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Arguably the most commonly used means of transportation to get to/from Beauvais is the shuttle from/to Porte Maillot. To date the price is:
Price: ONE WAY per person: €17 at the BUS TICKETS SALE POINT / €15.90 ON LINE
FREE for children under 3 years old (no ticket) // 3 years old+ 1day: 1 ticket
The airport webpage on airport connections mentions other options include TER trains from Gare du Nord, taxis, and a door to door shuttle service operated by SuperShuttle. Note that a taxi to/from Paris will set you back some 100-120€.