This is not mining, that’s the point 
The bandwidth and storage are used by real people with real data, thus no predictable income.
You can’t use the past usage to predict a future usage.
However, you can use this Community Earnings Estimator to have an idea:
So the better approach is to use hardware which will be online anyway, with Storj or without. Even if you would not have any usage - you will not lose anything, since it’s already online and all costs are covered.
The any profit will be a nice discount to your bills.
You will be paid $1.5/TB of used space per month, $20/TB of egress traffic to customers, $10/TB of egress audit and repair traffic, the ingress is not paid (you already paid for storage usage).