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.
I hope that helps.