Since cost of hosting is always going down at what rate are you estimating it to fall for your cost prediction?
I think the best you could do as look at the trend in cost reductions over time. Here is one recent source:
Notice for large data sets there was no change. So you need to look at your current data amount and go off of that.
Personally, I would be conservative and just assume it is not going to cost any less (it could actually start to cost more). If the differences in these projections is actually a significant amount of money your time might be better spent looking at non-cloud hosting solutions for a cost comparison.
I use this Amazon Calculator for Monthly Costs.
It's not infallible, but it's a pretty good guess.