American Airlines and United have round-trip flights from Chicago (ORD) to Las Vegas (LAS) for $97, NONSTOP. Flights October to December. Departures on Monday, Tuesday, Wednesday, Saturday, and Sunday. Very few extended weekends available. Origin/Destination reversible.
Carry-on bag is allowed for free on American. United charges a $35 fee for checked-bags each-way. You can avoid paying for seat selection by accepting the randomly assigned seat at the gate.