We can't tell you exact numbers because we don't know your bandwidth requirements and what you're using it for. Also, a VPS is almost certainly going to be shared and will always have a lower performance than a dedicated system. It's impossible to state clearly what kind of performance you can expect from such a system. Please read the system requirements document carefully and make an educated guess based on that.
https://openvpn.net/vpn-server-resources/openvpn-access-server-system-requirements/
If you can give us a lot of detailed information on how this VPN system is going to be used and what kind of speeds you want it to give you then maybe we can give you some rough idea of what you should look for.
