Data Center approved apps follow an annual subscription model, based on your Data Center user tier.
To determine the exact pricing for your team, see the Structure pricing for Data Center and Structure.Pages pricing for Data Center on the Atlassian Marketplace. Structure.Testy remains a free extension to Structure on both Server and Data Center platforms. Structure.Gantt is currently free, but will become a paid app starting with version 2.0. You can find more details here.