installation of open vpn on digital ocean linux server to give access to development server of programmer

We want to install an open vpn on our digital ocean linux server to give access of Development servers to Developers. this development server and server where this vpn will be installed has firewall and access control.
let me know following things:
- time and cost
- which open vpn would you setup
- how many connections can run on it for free
- what would be cost after free connections