British Airways Denver to Paris is priced around $500 to $550 round trip for flight departures mid-January to May 11. Google Flights shows low fares with Oneworld into June-August, but those low fares re-price in the $700s to $900s with American and BA on the final click. Searching BA website directly showed May 11 as the latest departure date for ticket prices in low $500s.
There appear to be a pattern of low fares in Icelandair cities like Boston, Chicago, Denver and New York to Paris and some other cities in Europe today. Paris is the only low fare I found to Europe from Denver with British Airways/American Airlines.
British Airways Denver DEN – London LHR – Paris CDG (BA) – Chicago ORD (AA) – Denver DEN