But don't worry - there is a way around this. And no, you don't have to spend hours (or dollars) building a mockup for your prospects before they've ever committed to an agreement. I've gotten burned doing that so many times it's not even funny. In fact, you can even SHOW your prospect THEIR website (in working order) without any labor or monetary involvement whatsoever. I'll show you how...
Ever heard of ThemeForest.net? If you're comfortable with Wordpress, this should be your new haven. They have thousands of top-notch quality Wordpress themes at around the $30-$45 price range. What's more, each of their themes come with full backend functionality, so you can easily edit just about any aspect of the site.
If I'm talking to a business owner and he/she is a little 'iffy' about taking the leap, what I do is offer to do a mockup of their new website, free of charge. Most prospects don't turn this down.
After they agree, I head over to ThemeForest, pick out a great theme for their business, and pre-load it on my laptop. The next day, I walk in, show off the design, explain what Wordpress is, how it can help them, and the various functionalities of the site. I take all of the risk off the table by showing them EXACTLY how their final product will look - and more times than not, this leads to a close.
Of course, you want to be up front and honest with all of your prospects. I explain to them that I don't code the website as they see it - I buy the 'framework' and subsequently design the layout, place their pictures, put up content, etc... This allows them to receive a cutting-edge, top of the line website for a fraction of what most high-end designers charge. They're happy, I'm happy, everyone wins.
Anyway, just a little tip I thought I'd share. Doing this has helped me close some sales and it's really the most simple thing in the world. Just get out there and try it.