Norwegian Air, a long-haul low-cost airline have round trip airfares travelling from Denver (DEN) to Paris France (CDG), nonstop!

Travel Availability:
Departs DEN: Nov 23, 30; Dec 7; Jan 11, 18, 25; Feb 1, 8
Returns: Nov 30; Dec 7, 14; Jan 11, 18, 25; Feb 1, 8, 15; March 8, 15, 22

Use dates and book it directly on Norwegian.Air [norwegian.com]. Since the only way to achieve this price is purchasing two separate one-way tickets, open two windows for the Norwegian site. One window will be DEN-CDG and the other will be CDG-DEN. Purchase two one-way fares, making it a round trip for $ 371.