Unity3d is overkill for a simple android game like that. I would just use a framework in that situation because it shouldn't be too hard to figure out.

You would have to read up on the unity licensing cost. I doubt you can use the free version for a commercial purpose. It could be up to $1500 I believe. Then the Google dwveloper account is $25.

It isn't going to cost you $10,000 unless you pay someone way too much to do it for you.

just posting to earn points.

If I was to develop an android game, simple game like Flappy Bird, using Unity3D (Free version) as an independent developer, how much would it cost me except for the developer account fee of 25$?

I searched this up on Google but all the result tells me that it would cost about 10k to make an apparently.