It's based on a lot of factors, but the "calculation" of now nice to be, and how much to charge, depends on:
1) How much competitors are charging and how they respond to changes (not easy info to procure, but the best way is to subtly ask the client)
2) How much respect and reputation you command with the client
3) Client's verbal and/or physical reactions and nuances when you discuss the price (this is an extremely tricky cat and mouse game; sometimes they will throw up false objections to get you to lower the price, but you have to see the bigger picture of how they're responding)
If you're well respected, you have more leeway to charge more, stick to your guns about the price, and to charge for change orders without damaging the relationship. It's a balancing act. I've lost clients for sticking to my guns too much, and also had situations where I could've gotten away with more in hindsight. I've found a fuzzy balance where I'll take care of a change in scope if it's 1-3 hours extra, but will charge for anything more than that.
To prevent low quality/low fee engineers from taking the project, you have to present yourself and your sole proprietorship very well. That's a big topic in itself, but generally means marketing, presentation, confidence, and talking the client through what you bring to the table and what pitfalls you will help the client prevent. But in some cases, the client absolutely doesn't care about the quality and will go for the lowest fee (usually homeowners or inexperienced clients). I personally don't take those clients because it's a two way relationship, but if you really need to keep the lights on, you can lower the fee to grab that kind of work.
I recommend you stick with engineering and not work at McDonald's. I know it's tongue in cheek, but the earnings potential is huge if you get the negotiation skills down. I suggest reading "Influence" by Robert Cialdini. I rarely comment about engineering on eng-tips these days; I accepted that I'm never going to be a great technical engineer, because I earn more money if I focus on business skills.