What is the etiquette for paying musicians at a memorial service?

this depends on whether the musicians are already receiving a stipend (payment) for their services. A decision to offer a gratuity is also culturally based, as where in almost every other country, stipendiary musicians would not be expected. The US, for example, has a propensity to expect gratuities for, well, pretty much anything, from pouring a cup of coffee at a counter, where the staff have merely done what they were employed to do, to the porter at a door, who was hired to do the same thing.

Using an extremely rough guide, approximately twenty percent of the fee would be an 'etiquette' based rule of thumb for the US. It should be handed to the conductor or lead, to distribute among the other musicians.

Outside of the US, stipends for a single musician, such as a pianist to organist, can range anywhere from circa GBP£50-150. (USD$75-200).