I dont usually play rebuys, but if I do I like to keep my rebuys + add-on around where the min cash pays out. So, if the min cash is paying out $6 is a $1 rebuy, then I'm willing to spend $6 on rebuys + add-on so I know I can at least break even. Paying less is great, but I wont spend more. I usually like to buy-in and rebuy immediately to double my stack.
It also depends where the blind levels are and the type of tournament it is. I want at least 50BBs when I sit down (buy-in + rebuy). More is great, of course. Also, it depends on the structure. I'd rather sit at a regular tournament with 12 min blinds with 50BBs than I would at a turbo tournament with 5 min blinds and 50BBs.
Another option I like to take with with rebuys is to wait near the very end of the late registration. Then buy-in, rebuy, and then take the add-on. So, if the starting stack is 2000 + 2000 (rebuy) + 4000 (add-on), you're starting with an 8K stack and it's after the rebuy, so you can play it just like a regular tournament now. Depending on the structure of the tournament and length of late registration, you're usually sitting around 35-50BBs and around average stack. So, you have plenty of room to do well, IMO. I find this method to be my best approach with rebuys if there is a good prize-pool to go after.