This picture is from the wiki.... it shows entropy drive mark I and III has having the same upkeep

the last one is probably 500 million and the wiki editor was the builder profession

i haven't been able to verify it myself yet, but chances are someone with builder profession put in the upkeep for the last one...
the unmodified upkeep is probably 500 M (the unmodified upkeep cost is supposed to be credit cost of the purchase divided by 20)

I am a builder and it shows 250M upkeep for me, so yeah... Normal unmodified upkeep is 500M and the entry was made by a builder and they forgot to de-modify the figure.