I am trying to build an OEM app and would like to know what will be the additional cost org have to pay if they reach their 10 GB storage limit.
This is something you should talk to your (or your client's) Account Manager for, but...
From experience, let's just say that the cost is prohibitive. Salesforce doesn't really want customers using Sales/Service/Education/Health/etc clouds for general data storage. I don't recall any specific numbers, but a little digging turned up $125/mo for an extra 500MB.
Long story short, you'll want to look for other cloud data providers (Google, MS Azure, Amazon, etc...)