Icelandair has round-trip flights from Denver (DEN) to London, Frankfurt, Amsterdam, Copenhagen, Helsinki, Dublin, or Munich (LGW/LHR, FRA, AMS, CPH, HEL, DUB, MUC) for $490 with a connection. Flights October to March. Extended weekends available.
Dublin, Helsinki, and Copenhagen are the cheapest in this list; in the $351-$399 range.
Frankfurt, London, Munich, and Amsterdam are in the $440-$490 range.
Some small OTAs on Skyscanner have this for cheaper than the airline. If you plan on booking there, beware of any site that charges a fee to cancel a ticket within the 24-hour “free” period. Use unknown booking sites at your own risk.
A Carry on bag (20-lbs @ 21.6 x 15.7 x 7.8 inches) and a personal item (15.7 x 11.8 x 5.9 inches) is allowed for free on Icelandair. You can avoid seat selection fees by accepting the randomly assigned seat at the gate.
Icelandair offers free stopovers, which good if you want spend a few nights in Reykjavik. More details are on their website (link).