I don't think, that you are free to set your own price. But of course, I am not current in these special law topics. But in Germany, as in many other countries: If it goes to court, you will loose against a big company.
It was an other situation, but a lawyer once told me, what happens when such things go to court: The corporation will bring 10 witnesses against you and will bring 10 experts certifying what the corporation needs.
And than you will be very alone and be gracious to get even a little money. You will loose your job anyway.
You're not free to set your own price, there are guidelines set forth in the law (ArbnErfG, § 11 Vergütungsrichtlinien) - which also means that the employer cannot undercut those guidelines and force you to hand your invention over for free. If there's a disagreement about the price, there's a mediation set forth in the law, so the first thing would not be a court case.
In general, german courts are quite employee friendly, so seriously, your employer would probably be at least as afraid to go to court as you'd be. Maybe not if it's a litigation-happy megacorp, but the bulk of german companies is medium-sized (Mittelstand) and they tend to shy away from court cases.
It was an other situation, but a lawyer once told me, what happens when such things go to court: The corporation will bring 10 witnesses against you and will bring 10 experts certifying what the corporation needs.
And than you will be very alone and be gracious to get even a little money. You will loose your job anyway.