To expand, remember that regular employment comes with way more 'compensation' than just the salary that a freelacing gig doesn't come with. As a regular employee, you get paid regularly, on time, based on hours, you get paid holidays, taxes paid, etc. A freelancer/contractor has much less certaintly and much more risk. Hence freelancers should be paid more per hour/day/week than a regular employee.
> Think about mockups/wireframes you can deliver.
Hint: Don't make the mockup look done. Use a mockup style that looks like drawing on a napkin (or even draw it on paper and send a photo of that paper). Non-techies think the amount of work depends on the amount of UI changes. If they see a pixel perfect photoshop image of a done web page, they could think that all the work is done.