Having said that, I feel the need to explain what I mean by this.....
You estimate (given the other factors) that you have a 2% equity in the prize pool if you rebuy.
That's not to say you have a 2% chance of winning the comp.
To clarify imagine a relatively simple payout structure where the top 2 pay on a 75%/25% structure.
At no point in the tournament will it be possible to have over 75% equity in the prizepool. (That's the most you can win)
If, at some point, you think there's a 10% chance of you finishing 1st, and a 10% chance of finishing 2nd. Your equity in the prize pool is
10% (7.5% + 2.5%)
If, at some point, you think there's a 5% chance of you finishing 1st, and a 10% chance of finishing 2nd. Your equity in the prize pool is
6.25% (3.75% + 2.5%)
I estimate the chance I'll parlay my one re-buy into a decent stack, and the chance I'll turn this decent stack into a cash finish. (to simplify things, at that that stage of the tournament, I'll assume my chances of finishing in each position is equal.)
So If I think I'll get a decent stack from my re-buy one time out of five, and I'll turn that decent stack into a cash finish one time out of 2, that makes my chance of cashing one in ten (or 10%).
So if we pay five prizes, I'll assume my equity in the prize pool is 2%, and go from there.
Obviously sometimes the chance of making a decent stack from one re-buy will be greater than one in five. I'll mostly consider the average stack when estimating this.
Similarly, the chance of cashing from a decent stack will not always be one in two. I'll mostly consider the number of players left when estimating this.
It's probably not that bad to assume it's always worth it. However, I prefer to make desicions concerning my purse strings with a certain degree of logic (or at least attempted logic), rather than faith that the value will always be there.