This isn't entirely true, for a few reasons.
Of course there is a finite amount, there is a finite amount of money in the world so it is impossible to have an infinite profit showing on your end-of-year's. However a few certain parameters make this a good business model for growth.
-The web is growing. More people are coming online, more people have faster internet connections and will continue to for a while.
- Internet marketing is growing. More firms all the time are using the internet model to sell their goods and advertise and will continur to for a while.
- The more that gets spent on the internet side of a company, the more money they will have. Because they are reaching a larger audience they are growing quicker than before. Therefore the more people who click on them will give them more money to spend on internet advertising which means they will/have to pay more for it.
Look at traditional advertising - a company might start with a small ad in a local paper, as that produces results they try for local radio, get more custom, go for a national paper/magazines and with more interest they end up with TV advertising before hitting S&S to run a multi million pund campaign for them.
However with the web, especially click-throughs there is less risk, more room for growth and better trends. It's also a lot easier to run a campaign while reducing the exposure to risk.
It is for the time being a pretty good business model.
As for the Google being hard to understand and scraping you for money. The problem is Google are a good salesman and they say "Not only will we give you the keywords you want but we'll automatically place you under hundreds of other relevant keywords and place you on thousands of sites throughout the world".."How much extra will that cost me?"..."Nothing Sir, that's part of the Google service".
All you need to do is look at the options and understand what they will mean to you bottom dollar. Spend some time researching and not jump in with both feet. Although there's plenty of secrecy in the 'auction' side of things, the options aren't that complicated as long as you spend some time evaluating and then monitoring them.