how much does adding a second floor to your one floor house... an estimate?

I don't know!

$50 million yen.

It's very difficult to give anywhere near an accurate estimate without knowing a lot more information, including, but not limited to;
Location
Size
Time frame required, etc

We have nothing to go on here other than one fact, its your second floor. Where do you live, will you do any work yourself, what is the first floor made of, slab or footing, wood or brick, flat roof or 12/12 pitch, bathrooms up there or not. You want ballpark, this is a big ballpark. Measure length and width of the second floor and multiply by $75.