It would probably be cheaper, yes. But first, things to consider....
How much are your termination fees? If you purchased a 3gs a year ago, you are in a contract with at&t and the rest of your family may be also, terminating that contract can be very costly.
Review usage, it's likely that you're paying 30$ per iphone for data, those plans have since changed (15 or 25 now depending on data usage). You may want to call at&t and find out if you could lower your plan, that may shave off some cost. Also, have them review your bills, it's not uncommon for there to be additional charges on your bill for things that you don't know you have and may not be using, so check that out. Lastly, find out if either of your parents employers get a discount through at&t, often good for 15 to 30% of your monthly bill, that could help a lot too.
So check it out first before you do anything drastic.
