I don't understand where bid directories are gambling ...
Users bid what they think a page is worth to them ... usually the minimum for a NEW directory. If the directory owner invests heavily and works hard to bring in a million visitors is the home page still only worth the minimum bid?
That is in fact the beauty of bid directories, you pay less to have a listing on pages with less traffic ...
and users decide what the page is worth.
Owners who re-invest their income in order to generate more traffic are rewarded with higher bids. That only makes sense, as the directory's popularity and traffic increases, pages are going to be valued more and bids will rise. You can choose not to add more to your bid and
your link will simply fall to the correctly valued page. Where is the gamble in that?
The advertising industry has been pricing by market reach for decades. You pay more for prime advertising space in newspapers and magazines. You pay more for prime time commercials and even more for the superbowl ads. That is the nature of the business.
I am sure there are some directories that are over-valued and some that are under-valued. It is really up to each person to evaluate what a listing is worth to THEM. Just because someone bids higher does not mean that you have to increase your bid.
Who knows, THEY may be cursed with some brain disorder, and YOU want to follow them. 
If you are not seeing any benefits then DON'T increase your bid!!!
By the way ....
I don't understand why someone would pay $299 or even $39 for listings 15 clicks off the home page. But that is my own opinion ... and I choose not to do it. :-)