Yep. And sometimes that buy up is more than the original ticket.

Travelling a couple of years ago MCO-ORD, we got to the airport several hours early. When asked if we could get on the standby list (with no expectation of being guaranteed a seat) I was told that I'd have to cough up $150 for each of our tickets just to be able to get put on the standby list. The RT tickets were only $140 to begin with. Kind of disappointing considering the GA told us that all earlier flights were practically empty.

Decided to pass on this. Fortunately MCO isn't that bad an airport to hang out in.