There's no money in the Culture, but Tier isn't part of the Culture.
Within the Culture money isn't used and most citizens have largely no conception of how money works beyond various truisms that are taught to young children about 'money implying poverty'. If you want something (within reason), you merely need ask for it. If it's being freely given you can have it. If that person says you can't have it, you can ask the nearest passing AI to make another one for you.
Money is a sign of poverty. This is an old Culture saying I remember
every now and again...
State of the Art: A Gift from the Culture
Almost the only aspect of her new life that she adapted to without
pause for thought was the total absence of money in the Culture.
That all being said, Tier (where the erotroupe are based) isn't part of the Culture. It used to belong to another race and was inherited by the artificial intelligences they designed to run the station. Various races are allowed to use the facilities and some maintain their embassies there.
As such, (not being a Culture world) money is still used to buy things.
This environmental diversity and the civilisational co-dependence it
implied and intermingling it encouraged had been Tier's raison d'être,
the very foundation of its purpose and fame for the seven thousand
years it had existed. Its original builders were, perhaps, unknown;
they were believed to have Sublimed shortly after building it, leaving
behind a species - or model, depending how you defined these things -
of biomechanical sintricate which ran and maintained the place, were
individually dull but collectively highly intelligent, took the shape
of a small sphere covered with long articulated spines, were between
half a metre and two metres in size and had seemed to have an intense
suspicion of anything possessing less of a biological basis than they
did themselves. Drones and other AIs were tolerated on Tier but very
closely watched, followed everywhere and their every communication and
even thought monitored. Minds were immune to this sort of treatment of
course, but their avatars tended to attract a degree of intense
physical observation which bordered on harassment, and so they rarely
bothered entering the world itself, sticking to the outer docks where
they were made perfectly welcome and afforded every hospitality. Tier,
after all, was a statement, a treasure, a symbol, and as such any
small discriminatory foibles it chose to display were considered