It is not how much the phone bill is, the key is the Monthly Recurring Charges (MRC).
Each and every line has it own MRC that is calcuated on a 3 month rolling average. So often just making a change today does not reflect in a magic adjustment in the upgrade eligiblity.
Your individual line has to have a MRC that is greater then 99.99 per month to qualify for a 12 month device upgrade.
This MRC is based on the line's individual rate plan + select features ( AT&T gets to select the features that are include ). Yes, the $30 iPhone data plan is considered a feature and it is included.
Also, for a FamilyTalk rate plan make sure that you subtract the additional $9.99 for the first add-a-line to get the true base rate plan.
Note: A FAN or National Account Discount is not part of the calculated MRC, so there is no negative to having a discount.
Summary: Always make sure your iPhone is the anchor line on a FamilyTalk rate plan! Dont' just assume, check your bill. This sometimes is switched when you renew a contract line on a FamilyTalk rate plan, that way the rep or agent get's the biggest credit.
Dave