Average people will try to build what the client wants and fail.
Good people will build what the client wants.
Great people will build what the client needs.
Be careful with pricing though. They want something cheap, that won't work. They need something more expensive that will work. Persuading them may be frustrating, may take time, may risk you losing the job to someone less scrupulous than you who puts in a lower quote.
Are you going to be paid for the consultation involved in persuading them to use the better solution?
Compare two other industries: sign making and electronic sub contract engineering.
If you ask a sign maker to make a sign with a spelling error you'll get a sign with a spelling error. It's up to the customer to know what they want, and it keeps a bright line of expected behaviour from the sign maker.
Similar with sub contract electronic engineering. They build stuff to your supplied documents. If your documents are wrong they'll build the device wrong. If they really cannot build the device they'll get in touch. They are charging you for a service and that's what they deliver. They're not charging you for design work, and so they don't do that.
Fixed priced projects only work in a few cases where you know exactly what you are building. They are a lose-lose situation otherwise, with the client usually being the loser.
Good. It is an opportunity to charge extra for fixing the mess. And if they don't make a mess - tough, you had charged too much in the first place.
Very few businesses are interested in getting paid once if they can get paid twice, so this pattern is perpetuated to the mutual benefit of everyone except the developers forced to spend 50% of their time building something they know won't work, and the other 50% rewriting it...