When similar expertise and experience are widely available (such as the ubiquity of web designers), what you pay for is no longer a supplier’s knowledge and skill, but their care factor.
A cheaper supplier will do a similar job for less. But they are also likely to care less.
At a glance, the outcome may look similar. But on deeper investigation, the supplier who cared less (and were paid less) will tend to leave subtle deficiencies. Spelling errors, leftover code that shows your site's been borrowed from someone else's, odd typography, broken links... Added up, such deficiencies may cause you to lose business to a more polished competitor.
Put another way: If you were unwell, would you choose a doctor on price?
How much do you care? Are you paying enough for care?
(Of course, just because a supplier charges more does not guarantee more care by default. And some decisions are made more on price.)
