Bear in mind that there is a difference between "regular" inflation and "Disney" inflation. Disney traditionally increases prices
far in excess of the Consumer Price Index (what most Americans are referring to when talking about "inflation" generally). Point being, when you hear about the U.S. having "low inflation" of 1.5%-2%, that number is irrelevant to Disney Parks & Resorts pricing.
I'd wait until the September 2017 rates come out, pick your desired itinerary and add 10% to it for 2018. 10% per year seems in line with general Disney P&R annual pricing increases overall.
Or if you don't want to wait, get a
DCL subscription to TouringPlans.com and use their FareTracker tool. Once you see how much your desired cruise cost upon release in 2016, add 20% (10% per year x 2 years). If it's a
little more than that, it's not going to be so much that it makes an affordable trip suddenly unaffordable. And like
@Jennifer777 said, if it turns out to be a little bit less, that's a happy surprise.
My guess is that that is the closest you can get in terms of estimates until the actual September 2018 itineraries are released.