Direct flights to the springs are slightly more money on both United out of o hare and delta out of msp, however it's not enough to justify the ground transportation costs from denver airport which is out in the boonies. There are however fewer flights, and they are on RJ aircraft and not main line. meaning less reliable, more lost bags, smaller seats if you are tall, and the window is for midgets.
If you buy one well ahead, Cos is sometimes slightly less than Den. For instance right now den is 296, cheapest out of msp and cos is 286, however the number of these seats is limited, the more normal cost is 520 or so. The airlines do not like to sell a bunch of seats to one entity at the discount fare.
Out of Houghton on united is two flights and the costs is almost 800 per person to cos while Den is 502. However the 502 is misleading as once again they won't sell 30 tickets to anyone at this price. Also don't forget out of msp you had to hire a bus from Houghton, for quite a bit, so the cheap flight isn't so cheap any more..
I could go on in great detail but you get the idea.
