Think sensible. M;ultiple technologies are multiple times the need of knowledge (sorry, good people specialize - yes, I do know java,from 10 years ago, but I do .NET ever since) and infrastructure (build servers etc.) AND time (yes, they may be similar, but there still is overhead - every bug must be fixed multiple times and tested etc.
A small vertical stack is superior from that. Once you hit a good income side you may decide it is worth pursuing multiple techs at the same time, but not at the beginning.
That said - I see .NET superior to java IF you can live with the mostly Microsoft only / Windows only environment (which still accustoms for 90% or more of the desktops - seres dont matter because every larger shop can host a .NET server). Why? It has a higher more widely used stack, which makes it easier to deal with it.
Costs are a non-issue - MS has great programs for the first years of startups. And later, sorry, the cost of a MSDN license per developer are small compared to a decent salary anyway.
This is 100% NOT a technology decision - it is an economic one. Every cost you put up has to have a justification, especially in a startup phase. I personally would try to build a stack where I dont have to hire multiple people to do what otherwise less people can be good at.