There's another reason why traditional economics aren't a good way to set price: people don't act rationally.

In fact, in many cases the decision to buy something is a split-second one people make before the parts of their brains that apply logic and reason kick in. To do this people take shortcuts, falling back on a judgment they've already made instead of making the effort to evaluate the value of something new.

Take a pair of loafers, for instance. It's hard to judge how much they're worth just from an online image. But someone who sees a picture of a teenager wearing the loafers along with cutoffs and a hoodie is likely to conclude they should cost less than someone who sees a picture of the same shoes worn by a briefcase-toting businessman.

Instead of trying to determine the value of the shoes-a difficult problem that would require the person to assess the material, the stitching and other factors-the customer substitutes a question that's easier to answer: How much would a high-school student or a businessman spend on shoes? That becomes the baseline for deciding whether the loafers are expensive or a bargain.

This phenomenon played out at Weebly, which offers tools that make it easy to build websites. The company on its website emphasized that its service is free and easy.

That made sense early on when the product was relatively simple, says David Rusenko, Weebly's co-founder and CEO. But people associate free and easy with lightweight and cheap. Small-business owners, Weebly's target customer, didn't want to pay much for the websites, but they didn't want their sites to look cheap, either.

Weebly has evolved over time into a tool with a lot of features that's still a great value. To make this clear, the company revamped its image. Its new tagline, "create a site as unique as your are," conveys a special product and its website is now filled with high-quality images from customer websites that look like they cost a million dollars to build.

"We used to use words that made sense to us, but they weren't the ones that made sense to customers," Rusenko says.

That's also why Libin banned swag. He doesn't want customers to associate Evernote with stress balls and 30-cent mints. Companies like Nike and Apple don't put their logos on things that aren't products, he notes. So if Evernote makes a T-shirt "it's going to be a really great one that you'd be happy to pay for," he says.

Similarly, he stopped promotions that let people get premium Evernote accounts at a discount, like a deal on MacHeist that sold a collection of software, including Evernote, for $29.

At the same time, Evernote extended its deal with Moleskine, which makes a $25 notebook that comes with a three-month premium subscription, because that product costs more than the subscription would on its own.

"If all you want is Evernote premium you can't get a deal," Libin says. "It's a way to keep perceived value from falling."