Open source alternatives are already out there. Wekan is one of the more popular ones. I'm not sure if it has all of the features of Trello as I haven't used Trello extensively.
https://github.com/wekan/wekan
Those features should take at most a month and cost at most $25k to build.
Yes there are other things that make Trello stand out but I don't see a justification for this purchase if not just to hire the team.