Making a decision based on "just one more customer" is a terrible idea unless you have unlimited resources at your disposal. If that's the case, congratulations on your early retirement and you may stop reading now. For the rest of us, we'll have to make decisions by weighing them against other options that you'd otherwise not be able to afford.
It happens all the time. A vendor pitches you with the idea that the money you'll spend with him will be more than covered by landing "just one more customer". Internally it happens all the time too. You tell your boss that the $12,000 PPC campaign you need will be more than covered by the $100,000 that "just one more customer" will bring (I'll save the idea that "need" is actually "want" in these scenarios for another post). Thinking about whether "just one more customer" will justify an expense is a luxury that none of us have.
Have you exhausted all the other better options already? Are your time, money, and other resources better spent on another form of lead generation? This type of critical thinking is completely left out of the "just one more customer" mentality. Ideally your analysis compares return on investment across the different options. When it's hard to track ROI, common sense works pretty darn well. Either way, spending money on PPC needs to be compared with PR, a referral program, other forms of advertising, or other options you can afford.
Justifying an expense with "just one more customer" is seductively easy. Deciding whether to spend the money elsewhere is not. Don't fall into the trap.
Another idea: how about we all just go out and get just one more customer without spending more money?