Sorry, rg, my understanding of the "benefit" performance is pretty much as yours is. We can even find it today, in the form of a "Nutcracker for UNICEF" or some other worthy cause. Special prices, special attraction, maybe dinner in the lobby or wherever, so much on the ticket tax-deductible (that's at least a relatively new wrinkle), and a stated amount of the proceeds to the beneficiary.

When Danilova retired, she had a real knock-down, drag-out fight with Serge Denham, who wouldn't give her a retirement benefit performance.

Anatole Chujoy clarifies the Russian Imperial Theatres' benefit system. (page 60)

Benefits were "given in honor of a dancer, singer or actor either as part of the contract between the artist and the theatre or to mark a special occasion, such as the tenth anniversary of the artist's being on the stage of the Imperial Theatre, his retirement from the stage, etc. All the money collected from the sale of tickets went to the beneficiary...."

Each ballerina and premier danseur -- 1 benefit once every two years unless otherwise stated in their contract (some dancers were allowed one a year)
Soloists -- 1/2 a benefit once every two years
Coryphées and coryphés -- 1/8th a benefit once every two years
The unit of the corps de ballet -- 1 benefit a year

Chujoy then goes on to describe how Nicholas Bezobrazov, a balletomane with talent at organizing benefits, collected ticket money, gathered subscriptions for a present for the dancer, assigned seats and organized a supper in honor of the dancer. The amounts of money mentioned make today's ticket prices look minuscule!