Check the price here: https://mockyapp.com/pricing/

Yes, a 30-day trial period for Mocky is typically available

For Data Center edition, you can extend your app trial up to 5 times - in other words, for up to six months. 
Extend your trial by generating a new evaluation license key from Atlassian Marketplace.

Follow these steps

1. Navigate to Manage Add-ons in the JIRA Administration.

2. Find the Draw Wireframes, Mockup, Prototype & Diagrams plugin.

3. Delete the existing license key.

4. Click the Free Trial button.

You should now have a new trial license for 30 days.

For Cloud edition, basically Atlassian does not allow the extension of trial licenses. However, in some special cases, it may still be possible. You can contact Atlassian Support directly for more details.