As others noted, there were three "Greek Drachma" currencies. The second drachma replaced the first after liberation from Nazi occupation, at a rate of 50 billion to 1. The third drachma replaced the second at a rate of 1000 to 1.
The 25 million drachma note is Nazi-issued, from the "first drachma". So it's theoretical "face value" converted to "modern" or third drachmai would be 0.0000005 drachmai, or a tiny fraction of 1 eurocent. Entirely theoretical, as Nazi-era and earlier notes were all demonetized.
Of course, it's worth far more as a "collectable note", even if its only worth a few dollars.
Don't say "infinitely" when you mean "very"; otherwise, you'll have no word left when you want to talk about something really infinite. - C. S. Lewis