I flew United to Telluride and they accepted the board bag as normal checked luggage. Not free but not oversize fee either.
Edit: Bring your boots as carry on, and from United's site:
United accepts one item of ski equipment per customer as checked baggage. Ski equipment must be securely encased in a suitable container. An item of ski equipment consists of:
Up to two pairs of snow skis and associated equipment in one bag and one ski boot bag. (Note: If ski boot bag contains other items in addition to or in place of boots, it will be subject to the applicable checked baggage service charge or excess baggage service charge.)
If applicable, the first or second checked baggage service charge applies to ski equipment.
Ski equipment in addition to the baggage allowance will be assessed at the excess baggage charge applicable in the market for a single piece.
Ski and boot bags weighing more than 50 pounds (23kg) that contain other items in addition to or in place of appropriate ski equipment will be subject to the applicable overweight checked baggage service charge.
The part in bold is what you have to be careful about. I didn't pack anything else in my bag other than board/bindings and hid pants and socks underneath in case they tried to open it to check, wore another pair of snowboard socks on the plane, including base layer, snowboard jacket with goggles inside the jacket pocket.
I think it's 25 for first checked bag (board), 35 for 2nd checked bag, so try to fit all your stuff in carry on including boots. Don't want them to lose those.