MullvadVPN is a VPN service with over 700 servers in 38 countries. They offers apps for Windows, macOS, Linux, iOS, Android and Firefox (Add-On), which are all open source on GitHub.
The pricing of MullvadVPN is very simple. The only offer one plan for 5€/month and there are no discounts for longer contract periods.
Read more on the pricing page of the service.