Exponential cost reduction. If the price of an MB of cloud storage halves every 2 years, then as long as this is true the total price never reaches two times the price of the first 2 years. So if 1 MB costs 1 SafeCoin for the first 2 years, the total cost never reaches exactly 2 SafeCoins:
2 years: 1 SafeCoin
4 years: 1.5 SafeCoin
6 years: 1.75 SafeCoin
8 years: 1,875 SafeCoin
10 years: 1,9375 SafeCoin
12 years: 1,96875 SafeCoin
14 years: 1,984375 SafeCoin
16 years: 1,9921875 SafeCoin
18 years: 1,99609375 SafeCoin
20 years: 1,998046875 SafeCoin
And so on, it never reaches exactly 2. Of course, the exponential reduction in cost isn’t constant like that in practice, but if you charge for example 3 times the price of the first halving period, then reality is allowed some flexibility. 
Anyway, the idea of a fixed cost for “infinite” storage is mathematically feasible, as long as there is an exponential reduction in cost.