Iphone 3g Data question

[puzzledazn]

Okay guys.. I've been wondering this. If i get the iphone 3g the data plan cost $30 a month... I do know someone that works with att, thus if i can change the internet plan to liek $15 a month or even cheaper... and assuming that 3g is available in my area, will the internet speed be the same or will it really not be 3g unless it is on the $30 month plan? THanks in advance!