Haha... yeah! what you said is buying domain extension not domain.
.com, .net, .org etc are domain extensions.
where something.com, nothing.net, company.org are called as domains.
From your question I understood that you wanna buy a new domain extension instead of domain and to sell the domain of your extension. Of course this is a good idea.
As you said drink.coke domain will be costlier than
.coke extension its false.
There is an organization for it called ICCAN (Internet Corporation for Assigned Names and Numbers) You can contact ICCAN and buy a new domain extension from them. New domain extension costs $50Mn from that you can derive unlimited domains and you can fix your own price. Per domain registration you have to pay $0.10 as ICCAN fee.
But this is a great business. If you purchase a smart extension you can earn millions of dollar by selling domains.
Hope this is what you want...
All the best.