Unlike many software out there, with Crossover, you don't buy a specific version.
First, you buy a licence which come with support and updates/upgrades for a specified time. During that time, you will have access to all updates or upgrades, even if it is a version jump, from say 12 to 13.
Second, should you choose not to renew your licence, you keep the right to access the latest version you had at expiration. Of course, should Apple change something big which kills the version you have (as they sometime do), that means you have to buy again.
Third, renewals are cheaper than the original pruchase, but since that could change eventually, I'll let you look it up when you'll renew.
As a custormer (and advocate), I also wish to underline that Codeweavers is NOT like other companies out there. They have a strong tendency to amiable resolutions to any problem you have. I find the licence to also be one of the fairest out there, and you can read it here.