If the buy-in is cheap enough, and the guarantee good enough, and the add-on big enough, then either figure a very late buy-in or 10x the buy-in as the cost to get to the 2nd phase of the game, which IMHO more closely approximates decent poker.
I avoid rebuy's for the most part these days. On-line or live. In live rebuys, I can't compete with folks who have $1000 wads in their pockets for $40 games, and I wouldn't have believed that was the case if I hadn't seen it happen more than a few times. Re-entries are a bit better, mainly because one has to get up and go re-register for the tourney, and in those few minutes they have a chance to re-consider.
A local casino has free entry tourneys, with $10 fees and almost unlimited rebuys at the table, up to 4 rebuys at a time. This game starts with a $3K guarantee, and never fails to cover that guarantee. Seldom more than about 90 players means each player rebuys, on average, 3 times and buys the add-on.
'Free' seldom costs so much.
I was regularly doing the cheap online rebuys, until I started live and realized that my original foray into online poker and tourneys were fueled by the limited risk nature of tourneys. Those pure limited risk tourneys are becoming rare these days, and trying to use that limited risk mentality by promising myself I would only do the original buy-in and the add-on, usually cause me more frustration than I care for.
I do expect all tourneys, both live and online, to become re-entry games, which will make the casino's those extra tourney fees, in the very near future.