It sounds like she was used to working in a very specific fashion and didn't enjoy changing that up. At $300/hr I'm sure she's used to getting her way, so having to "compromise" isn't something she was willing to do, despite the money. It happens, and it's unfortunate. I come across this a lot in Project Management.
I'm not really sure how much a typical interior designer commands, but I'd consider $500-$1,500 low, but that also depends on how big the house is and how involved they are. I would imagine most are independent workers, so I'd expect their rate to cover a decent salary @ benefits, so I wouldn't think that $50 an hour is unreasonable. At that rate, $500 only buys 10 hours, and $1,500 only buys 30, which is about 3.5 days of their time. Doing design and being there in person on a build I'd probably expect at least 1-2 weeks of my time, but that again depends on the overall size and what they're offering. Are they doing mock-ups/sketches, or are they just picking a materials list for each room?
I'm just now getting into interior design myself on the side, and currently focusing on the design/mock-up aspect to give myself an edge, should I jump into that pool and finally decide to stop working for "the man."