It's been under some form of heavily active development since 18 November 2011. (though it was in development by Notch since 2009 and he quit his job and lived on the revenues at that point)
At that time there were 25 employees, all living in Stockholm Sweden, the cost of a developer there in 2011 is roughly 60-75k SEK/mo (lets say 60).
(25x(60x12))x9 = 15M SEK in Salaries.
Not including: marketing (usually more than half the total cost of making a game), build servers, distribution servers.
To put that into context: Final Fantasy IX is on the list of most expensive games ever made and cost 40M EUR.[0]
[0]: https://en.wikipedia.org/wiki/List_of_most_expensive_video_g...