How much will data plan cost on Sprint with replacing droid with iPhone?

I have a Galaxy S2 and want to replace it with an iPhone. I have a year left on my contract with Sprint and want to get an iPhone from Craigslist, not sure which one yet. I'm on a plan with my parents and have unlimited data, if I buy an unlocked iphone and replace the droid I have will the data plan have to change and the rates go up? And will the price depend on which iphone?

No the price will remain the same, aside from insurance costs. I imagine, since you are buying this used, that you will probably not add insurance. This may in fact, bring your bill down as you might have Sprint's Total Equipment Protection insurance for Androids. So no, cost of the bill will not go up.

Word to the wise though. Meet your Craigslist seller at the Sprint store. I've seen a lot of people ripped off that way because they end up buying a stolen phone. If you meet them at the store, you can first have the representative check the serial number. Ask them to activate it for you and if it all checks out, pay the seller after it has been placed on your account. It shouldn't take too long, but will save you from anything they might be trying to get away with.

Good luck. (Also, stick with an Android. Go Google! Lol)