Lightning Experience is included in your subscription to Salesforce. However, there may be some dollar and time costs associated to migrating. For example, if you have custom visualforce pages or code, custom buttons, or other custom solutions, those may need to be updated/converted to work with Lightning Experience.

Generate the free Lightning Readiness report in your org to see what the level of effort would be to convert. You can access it by navigating to Setup | Click "Get Started" in the Lightning Migration Assistant in the upper left | Check Readiness | Check Readiness

This will generate a PDF report full of details and let you know what areas of your org may need to be updated to work in Lightning.