Hi,
I have seen providers as low as $10/mo and as high as $40/mo.
Some important things to consider would be:
1) Encryption level. 128bit is the lowest I have ever seen anyone provide, but 2048bit is really much more secure.
2) Bandwidth that is included. Some of the low cost providers include a very very small amount of bandwidth, and then try to upsell you to a higher priced package.
3) Reputation is important as well. I have heard of companies "snooping" traffic and that of course is not what you want.
4) Is the IP address dedicated to you, or will it change? This may or may not matter to you. Many people use their VPN's to have a dedicated IP so they can lock down their servers or services to only allow access from their VPN IP address.
I hope that helps.