For domestic on United I end up paying $100 each way (consistently) since the hard case weighs around 65lbs with bike and shit inside. I have MileagePlus Silver status which gives me a fee buffer for the weight between 50lbs and 70lbs. That means I only pay the "over sized" fee of $100 instead of over sized + overweight. Once I've had to pay $135 - cost of a checked bag + over size fee, but most of the time I've found if you pay the $100 ahead of time online when you check in online on United website, and the agent sees that you payed an over sized or over weight charge, they just push it through.
Again, every United agent interprets their rules different. But I've found that on every airline. The only exception has been Southwest, but even had a ton of issues with one agent when going to Texas in April - tried charging us $150 for each bike. So, no airline is clear enough apparently. Be prepared for a fight - always.
What I started doing is saving my United baggage receipts and have presented these in future arguments. With United, and other airlines, I've won "the fight" by being able to prove what I've paid for bike shipping in the past.