That's the thing with affiliate marketing, you find a niche that's unsaturated and develop it into a site that makes a little bit of money. As matty said, with some work, it could easily reach high up on page 1 and bring in traffic and hopefully some commission.
I'm not saying that yellowtrousers.co.uk is a good name, cos it's not, it's actually pish. But if you work on it, or any other keyword domain people think is not worth investing their time or money in, you could easily recoup your costs because no ones bothered to enter that market thus little competition. That's how you corner an untapped market. Simples.