Established ensembles often have fixed rates, and university or student events usually receive a discount. These associations are registered and can issue invoices. They do not pay VAT or income tax.
If you book a more temporary group instead, you should expect to pay a fee to each participant. Hiring a smaller part of an ensemble rarely works out cheaper – the main costs are for direction, sheet music and rehearsal time. Also keep in mind that acoustic groups, such as choirs, vocal ensembles and string quartets, need good acoustics to be heard. Hard surfaces above, behind and beneath the ensemble make a big difference.