Visual Website Optimizer starts at $49/month: https://vwo.com/pricing/#web-testing
If you don't have developer resources dedicated to your team, you may find yourself unable to run some tests, and you'd have to upgrade all the way to 'Enterprise' on the 'Conversion Optimization Platform' to get a Dedicated Account Manager, who would, in theory, help you tackle more advanced topics and test designs.
Any way you slice it - as soon as you hit moderately complex A/B testing the price goes up quite a bit.