How is it unethical or even able to make people uncomfortable. if you're self employed you choose ALL variables for taking a job. Lets say I'd do £500 a day contracting work in London, but if they wanted me to work in Glasgow, I'd quote £500, + travel, + per diem + expenses/accomodation. So lets say I'm asking for £700 a day or something instead, if they want me thats my new fee.
If one job was particularly big and would need more support, I would charge more.
Working with difficult clients is merely another variable, I don't really want to do X, and amount Y extra would make me put up with travel, type of job, personel, whatever, thats life.
IF said person was easier to work with, they'd be charged less, its THEIR behaviour that is costing them more, not yours, I'm fine with that.
In a huge number of situations in life people undervalue their work and would have found the people they work for would have paid significantly more for the same job, is it unethical for them to offer less because the person isn't aware just how good they are or how big a job it is? Thats life.