Storage costs are certainly something to be weary of, and something I have always hone through great efforts to minimize.
We have just over 5GB of total storage in our org and are only using ~45MB after a year because we planned out model around both storage efficiency and a sane data model.
You don't use fully normalized database designs in Salesforce if you can avoid it.
You are leaving out storage costs. There's a minimum record size and then per licence billing, something like $250/GB/month.
If anyone needs some info/consulting - PM me via email in profile.