More often than we'd like to admit, people talk about the cost of developing a website (or any software product) rather than the potential gains. I think this tends to lend itself to the argument that people feel they need a website almost for the sake of having a website - rather than actually working out the goals of the website that they are commissioning to have built.
Having been working on some pay-per-click advertising campaigns recently however, its clear that by spending just a little time on optimising your website you can save a lot of money on click costs. It's well known that Google recognises the subject (and therefore search relevance) of a website, but by ensuring that the meta data and page content is relevant to your pay-per-click campaign, the cost to get to the top stops is significantly less.
What's more - if you visitors land on a page relevant to their search, they are more likely to stay on the site (rather than navigating away immediately - so not only do the click's cost less - they end up being more effective too!
Plus (yes there is more) that relevant copy should actually help you to improve your organic search listing position for these searches too!