RE: "Free mockups" - covered well here, as a freelance provider, here is my take on it.
People looking for work have been trained by the Internet to look for "free," if they can't find free, expect quality for very little or free. There are scores of e-books out there instructing how to "work it" so you can get work for free, or very near to it. You can spot these right off:
"Should be a simple job for someone who knows what they are doing" - playing on your ego to prove yourself.
"Before I accept a contract, I want three mockups" - very likely which will wind up in the hands of the one they select to do this job, you get . . . nothing.
"This shouldn't cost more than $100" - even if in reality it's $1500 of work.
The problem, really, is that there are many "providers" out there for which it's not a business. It's just extra beer money, or they work out of their homes for extra money, they don't have to follow the ethics of those trying to run a business. They have no overhead, no business expenses, and their only directive is to make that $50 at all costs.
Many buyers are looking for these types of providers. Their words say "I want a professional" but the words between the lines say "I want someone 'hungry' who is willing to undersell their talent to my advantage."
Here is what a high rate per hour does: it weeds out these types of buyers. Sure, you will get less inquiries, but they will be serious ones who are more likely willing to pay what it's worth.
The second bit of advice: do not negotiate price. When you propose a project, one of two things happens. The buyer accepts your pricing based on your comprehensive proposal that justifies the pricing in every way. If the buyer begins to negotiate price, it's is a clear sign that they are seeing how far they can manipulate you. Reduce the price because you fear losing the contract, and the message is clear: you can be "worked." The project will continue on and will have various scope creeps. Add this feature, add that feature, and it shouldn't cost me any more, which is supplemented by an insulting jab: "this shouldn't take long if you know what you're doing."
Overall, buyers feel that are in control because they are stringing you along with the dangling carrot of being paid. Take away that control by sticking to your guns, and let go if you have to. If you lose the project, you're likely better off.