0.01S+max(0,7.20*(R-0.0017S)/4)
S is number of GB stored
R is biggest retrieval in the month
4 is the average number of hours a retrieval
For an example with 10TB storage (replace 10000 to change): http://fooplot.com/plot/4pu7u2gpox
x is biggest retrieval in GB, y is $/month