If you have this relationship:
Reservations::ReservationCode = Services::ReservationCode
then this calculation in a field defined in Reservations:
Sum ( Services::Price )
will compute a total price for all Services linked to that reservation. But I suspect that this is more complex than that. I suspect that each time you create a reservations record, you need to select specific services to link to that reservation. This becomes a many to many relationship and you need a related join table to list your selected services for each Reservation:
Reservations::__pkReservationID = ReservationsLineItems::_fkReservationID
Services::__pkServicesID = ReservationsLineItems::_fkServiceID
With this set up, you put a portal to ReservationsLineItems on your Reservations layout. You format the _fkServicesID field with a value list for selecting services. If you enable "allow creation of records via this relationship" for ReservationsLineItems in the Reservations to LineItems relationship, you can add new records to your portal just by selecting a service in the _fkServiceID field.
A Price field can be defined in ReservationsLineItems that uses an auto-enter field option to copy over the price from the matching Services record. Then Sum ( ReservationsLIneItems::Pric ) will return the total services cost for a given Reservations record.
Thanks, it was what I needed to know