More info about these domains and google apps accounts:
Google apps allows you to use Google as your email host for your own domain name. That is you get a gmail like interface as well as pop3 and imap support. These are full fledged google accounts which you can use for gdrive, google talk etc.
After you buy one of these domains you can add as many as 600 domains to the google apps account and create accounts at any of those domain names.
The domain name with the google apps account can be any TLD. It has no bearing on the TLDs of the other domain names you can add to the google apps account.
The number of licenses tells you how many accounts you can create or how many users you can support. Accounts have their own username and password. However, email aliases do not count towards your licenses limit and you can create unlimited email aliases.
Huuuge price drop because google has made a game changing change. You can no longer create independent accounts at secondary domains. Secondary domains can now only be added as aliases. See here for an explanation