Yes, you can keep it on the same plan, but you have to wait until your 2 year contract is up with the Solstice, or else you can't buy it. The highest data plan is 45 dollars a month, which really is the only one worth getting if buying an iPhone. That is not including texting or calling minutes. Those would add another 20 dollars for unlimited texts and 70 dollars for unlimited calling. GPS would be another 10 dollars a month, so it will be a lot more expensive. I wanted the iphone but my parents didn't want to pay the 45 extra dollars a month so I got a Blackberry (torch) instead which is only 15 dollars more a month!
hope this helped!