I got sucked into clicking on an ad Frontier Airlines had today for 90% off flights. After fiddling around with it for an hour I finally found a flight that qualified and saw it was saving me about $7.
That is because the 90% is off of the fare only, not the taxes or fees. So the $75 normal price only went down to about $68. Then you have to pay for bags, to pick your seat, to board early, and other options that you may want, and you don't even get to fly on the dates or times you prefer.
Long story short, 90% off seems like the deal of the century, but it is actually too goo to be true!