Thanks for your question, are you looking to use the VPN to protect emails on your apple devices?
Generally most VPN’s are going to be around $60-$100/year and support several devices. Reliability is often based on your cell carriers service as the VPN rides over your Internet connection, if you have bad coverage, the vpn will have issues staying connected.
We like expressVPN, Nord, IPvanish and pureVPN as they work on a variety of devices, are easy to use and are reasonably priced.
In regards to email are you using something like a free gmail account, or a custom domain on Office 365, or an ISP’s old POP or IMAP account?
For most modern mail providers, your email is encrypted between your mail client or browser and their server, for both sending and receiving. For these a VPN is not needed to keep your email private.
For some older types of email providers it was possible to set them up using no encryption which allowed anyone on the networks from your device to the mail server to be able to read these messages in clear text. For this a VPN would help for your side but a better option would be to switch to a more secure email provider.