Theory behind rebuys/addons

So I was watching a bunch of donks play a televised SNG which allows rebuys. The structure alternates between winner take all and the more standard 50/30/20. They had an option where you could surrender your stack if you rebuy and at the end of the rebuy period. So the guy re-bought after a casino employee convinced him that 10BB is greater than 5BB. But I still think he got fleeced b/c there was like 8 people left and 10BB at that stage can't be worth anything close to your stack at the start of the tournament. I know chips get more value the deeper you go, but yeesh 10BB is nothing if you're not close to the bubble or a big pay jump.

If you really want to figure out if late-registration/rebuys/addons are worth it, how do you go about doing that? Do you just compare it to the value of chips at the start of the tournament? Someone smarter and more math savvy please enlighten me, or at least point in the right direction.