Look at the online competitors for your potential business. In the wacky world of organic search traffic and PPC advertising, this means more than just companies that directly compete with your products. Indirect competitors may dominate the search results and jack up the PPC costs. Those indirect competitors may also have different business models such that it is economical for them to pay much more than you can for keyword clicks. Their profit per sale may be higher, their conversion rate may be better, or both. They may have also gamed the search SEO environment so that they dominate the SERPs for a huge number of keywords. If several large companies have spent ten years building an unassailable mass of backlinks, much of which may come from their own private blog network or other online properties that they own, then you are pretty much screwed when it comes to getting a good SERP position.
The reason why a lot of people who set up money sites on the Internet choose fringe niches that have a low number of exact search matches per month is because it is too hard to rank a site for a popular (lots of broad searches per month) keyword with strong competition.
"Listen, my son. Trust no one! You can count on no one but yourself. Improve your skills, son. Harden your body. Become a number one man. Do not ever let anyone beat you!" -- Gekitotsu! Satsujin ken