The Savoy chapel is open to the public and is part of the Duchy's London estate. The Duchy of Lancaster owns considerable property portfolios (including many historic properties) outside Lancaster and is obliged to keep them in good repair.
I'm not sure where your figures come from, but this is the statement in the Annual Report for 2025 (overseen by the Treasury and audited) -
"Until 31st March 2025, net income from bona vacantia, after
allocations for future liabilities and the costs of administering
bona vacantia, was applied by the Duchy to the costs of
Palatine administration and historical obligations, which include
Lancaster Castle, The King’s Chapel of the Savoy and nine
other castles, and at the direction of the Council the balance
was transferred to the Duchy of Lancaster Jubilee Trust, the
Duchy of Lancaster Benevolent Fund or The Duke of Lancaster
Housing Trust, and other separate registered charities.
From 1st April 2025, the net income from bona vacantia
will be transferred on receipt by the Duchy to The Jubilee
Trust, (now renamed The Duke of Lancaster’s Foundation)
after consideration made to any further contributions to the
Late Claim Fund and any direct charitable grants made
by the Duchy.
The Duchy of Lancaster charitable funds comprise three
separate registered charities.
•
The Duchy of Lancaster Benevolent Fund (established in
1993) makes donations to a wide range of charitable causes,
primarily within the County Palatine. In the year to 31st March
2025 it made 273 grants totalling £473,000. At 31st March
2025, it had an endowment of £18.4m.
•
The Duke of Lancaster Housing Trust (established in 2007)
provides affordable rural housing, predominantly within or
close to the Duchy’s estates. At 31st March 2025, it owns 43
houses, with the most recent asset valuation of £7.9m.
•
The Duke of Lancaster’s Foundation (formerly The Duchy of
Lancaster Jubilee Trust) was established in 2001 to benefit
charitable causes in all areas associated with the Duchy and
will, from 1st April 2025, be the recipient of all net income
from bona vacantia. The Foundation had an endowment of
£38m at 31st March 2025.
The annual audited accounts of all three charities are published
separately and are available from the Charity Commission and
from the Duchy Office."
The accounts clearly show that the proceeds from bona vacantia equal the charitable payments. At no time has any of this income been used for the benefit of the monarch!
The DoL also has signifcant investment income and rental income from its wider estate (particularly it's commercial and agricultural estate etc). It is this income (net of expenses), not the bona vacantia income, that goes to the monarch.
The attached gives the breakdown of the proceeds of bona vacantia for the last two years. As I said, I have no idea where you got your figures for the Savoy Chapel expenditure from.