I have been trying to do $300-$350 round trip BWI-DEN after taxes on southwest. If your days are flexible I’ve done ~$200 before, but that might have some odd days/hours for your flights.

recently I’ve seen southwest does sometimes include that route in the $69 tier when they run sales. So if you are lucky you can go even lower

My son and I snuck out to DEN mid-week last month, 2 weeks before xmas.  $130 ea roundtrip! The BWI-DEN nonstop is cheapest, espcially the early flight, which gets you to DEN in time to be on Summit county slopes by 1, or WInter Park a little earlier. 

I bought my ticket 10 days out at $230 and changed flight (the app/site lets you "change" it to the exact same flight) when the price dropped a few days later- SWA so no fee.  This was ~6 days out.  Their pricing can be very dynamic,. especially if you look at Tue-Wed-Thu flights.

Right now I see $59 out Tues the 2nd- $105 back on Sat 6th.  6:20AM out, 7:40PM back.  Gets you Tues and Sat as ski days!

I usually fly from DCA on Frontier, paying around $125 - $160 round trip, before fees. I've never flown with my skis, so I'm not sure how much it costs to fly with equipment, but for ~$70 each way you get checked and carry on bag.