Create App from Salesforce Community Portal

Description

We have a Community Portal built within our Salesforce environment, providing our clients with access to key information and some other small functionalities. The Community Portal is browser based, and is operated through Salesforce's 'Customer Community - Enterprise Edition - Members' licences. We tend to have anywhere between 20-25 active users at a time.
We would like to turn the portal into a downloadable app available both on Android and Apple. We are not sure what this work would encompass, therefore please provide a detailed proposal with different stages and suggested pricing.