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:
Old New First 1 TB $0.150 $0.140 per GB Next 49 TB $0.150 $0.125 per GB Next 50 TB $0.140 $0.110 per GB Next 400 TB $0.130 $0.110 per GB Next 500 TB $0.105 $0.095 per GB Next 4000 TB $0.080 per GB $0.080 per GB (no change) Over 5000 TB $0.055 per GB $0.055 per GB (no change)
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.