Herewego, It's not always so simple.

What if you do a great job and someone comes along and offers to do as well for less money?
What if you do a great job but you're extremely difficult to work with?
What if you do a great job and the client doesn't realise it?
What if you do a great job but ultimately aren't charging enough for your services?
What if you do a great job, but the client is an idiot?
What if you do such a good job, the client decides s/he can hire someone else (a relative, say) to follow your lead?

Yes it's probably true that it's cheaper to retain clients than to find new ones, but if you're not spending much on marketing (say, under $15/client) to begin with it's not such a problem.

But the idea that a successful business needs repeat customers only holds when your profit comes from volume. If you sell something that is consumed quickly or has a limited shelf life (web content, ball point pens, snow peas). For this reason, and because there are many people out there who could provide these commodities, keeping your profit margin low brings you more work. 100 clients, each with work for your every month, might keep you more than afloat.

On the other hand, what if you provide a service or commodity that's only needed every once in a while (a mainframe, a desk, a refrigerator)? In such cases there's probably less competition for your work, and you probably have a higher profit margin, but 100 clients, each needing your services once every 3-5 years, may not be enough to keep your business afloat. You would be more dependent on good networking and marketing than on providing goods or services to any one client.

In short, how you retain clients and what that means to your business depends on your business model and, to a lesser extent, on what you do.