I follow your math, but not your conclusions. How is one success in eight, "even money"? The odds of one or more success in eight trials is 1 - (0.8^8) = 83.22%. Those odds are much more than even money, are they not? i.e. I wouldn't bet money against having at least one success in eight trials. In the long term, you would lose all your money.

By your math, "even money" would be the odds of one success in three attempts (1-(.8^3)=51.2%

the term "even money" is used when the chances of one event happening is roughly equal to that of another event.

By my math...

the odds of seven consecutive failures is 20.97% (0.8^7=.2097)

and the odds of any one attempt being a success is 20%.

The chance of either event happening is roughly equal, hence "even money"

So, getting one success is eight attempts is roughly a 50/50 proposition.