Add-on question

[naphinfitos]

Ongoing debate between me and a couple of friends.

When is it correct to not take the addon? The specific sitaution was in a $3+R qualifier for the Sunday $215 on pokerstars. I forgot to rebuy before the tourney started, and tripled up first hand. By the end of the break I had a stack of 13,000 which was double average stack. I elected not to take the addon, because I had only invested $3 thus far and didn't think it was worth doubling my investment to gain only 2000 chips. My friends claim that the rebuys are a sunk cost, and that I was wrong to weigh the fact that I had only invested $3, and that I should've rebought. Basically their arguement claims that if I had spent $30 to get a stack of 13,000 or just $3, I should rebuy either way.

What is the correct strategy for rebuys, and is the amount I spend to build my stack a sunk cost?