You should add-on just about every time. If you're in first place with a ridiculous chip lead, like you have 60k and player in 2nd has 30k, then I can understand your doubts about there being any real benefit by adding on, otherwise its a simple call.
Anytime there's an edge you should take it w/o hesitating. I don't really think the buy-in matters much in this conversation because you shouldn't be playing above your BR anyway, and if the price for the add-on is a lot for you then you don't belong in this tourney to begin with.
If you give a look at some high stakes rebuy tournies like the $109 ones, or the $215, you'll see that even the chip leader will add-on every single time, even when he has, for ex, 19k and player in 2nd has 11k. The reason for this is pretty clear, and no, its not because he's rich or he's a winning player or the $109 means nothing to him.
Sorry dj, you're wrong in this one. I understand what you're trying to say but its not the only way to look at it. The make or break attitude by some players after the rebuy period is not a reason you shouldn't add-on, and TBH the play actually gets much better after the break. I might share some of your thoughts on this, but honestly I don't see how not adding on can actually help you more than doing so.