10m
Mycroft and Sherlock come from a family where almost everything goes to the eldest son, so Mycroft inherited nearly the entire Holmes estate and wealth (although Sherlock has a nice trust fund).
This is tempered by the fact that Mycroft ensured that the Stradivarius violin, which is a family heirloom, went to Sherlock.
There is also a country house somewhere that is co-owned by the two brothers - Mycroft could arrange that, because it had only belonged to the family since the early 20th century.
