Couldn't figure out how to copy the hand from HM2 so I will type it.

iPoker 200k Sunday Special
158 people left, 153 will cash.
I'm new to the table.
Folded to me on the button with A9s. I got 21 bb.
SB: 28 bb, BB: 9 bb.
Whats your play?

Anyone have any suggestions of how to think on the bubble?
I know a good way to think about knockout's is that u get x amount of extra chips when calling an all in from someone u cover. Can somewhat similar thinking be applied to bubble spots?