Probably worth very little. Post WW1-Germany went through a period of ridiculous hyper-inflation. Per Wiki, in the 1920s people were using 1M mark bills as notepaper and a 50M banknote issued in 1923 was worth $1 at the time. Within weeks it was worthless due to inflation.
I’d hazard a guess that an organ-maker who stuck a 500M note into a time capsule was burying something from the time when that would have bought you a loaf of bread.
In 1924, the mark was revalued at 1 trillion to 1, so 500 million of the old marks would have been worth some fraction of a cent (exchange rate was roughly 4 trillion to 1 USD.) This is certainly the case given the context.
In 1930 when the money was hidden, the exchange rate for the new currency was about 4 to 1 USD, so 500 million would be an unimaginable fortune. Not sure what $125 million in 1930 dollars would be today.
I remember reading somewhere that intact US dollars of any era are legal tender in perpetuity. Would Reichmarks even be honored today? The government(s) that issued them no longer exist(s)!