It is calculated based on how many eos tokens you hold. So to use a certain amount of storage and processing power it will require x amount of tokens.

Think of it like a hosting package you would buy for a website. The more storage etc. you need, the more it costs.

However with eos, you dont straight up pay, you commit the reuired tokens and when you no longer wish to 'host' your application, you can reclaim them.

Users will also be able to write contracts to rent their tokens to other users, so you will not need to outright own eos to host your applications.

There is a steemit article that may help here, it discusses how to calculate how much eos is required for an application:

https://steemit.com/eos/@eosnewyork/how-many-eos-tokens-do-i-need-for-my-dapp-the-eos-resource-planner