Icelandair has round-trip flights from Chicago (ORD) to Helsinki (HEL) for $366-$397 with a connection. Flights January to February. 7-day minimum stay required.
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.