There is already a <buy><extra>1234</extra></buy> But its not documented what is meant here exactly. This could either refer to the implied extra costs (A) that would occur if it where rented out, or the costs you are mentioning here (B), yearly costs for maintenance and operations. It could also refer to some hidden fees that are added on to the salesprice (maibe VAT)? not sure though.

As far as I know this field is not in use yet so we could define it now:

Possible definition for current implementation interpreted as (A).

• Don't think that makes much sense, since we could instead allow prices to be defined even for type=buy.

Possible definition for current implementation interpreted as (B).

• existing: price→buy→extra: Refers to the extra costs (operational and maintenance) per year that this property costs when owning it.

Possible definition for current implementation interpreted as (C).

• existing: price→buy→extra: Refers to additional fees/costs that go ontop of the purchasing price.
• new: price→buy→yearlyOperationAndMaitenanceCosts: Refers to the extra costs (operational and maintenance) per year that this property costs when owning it.

i would make it rather explicit like you proposed with:

new: price→buy→yearlyOperationAndMaitenanceCosts: Refers to the extra costs (operational and maintenance) per year that this property costs when owning it.