The content cannot come from outside
A salon's best content is the work itself — a colour result, a finished blow-dry, a correction that went well. That happens in the salon, unpredictably, and it has to be captured by whoever is standing there. An agency shooting monthly produces a handful of staged images and misses everything that actually happened.
The workable model is a stylist or receptionist with a phone, fifteen minutes a day and a simple habit: photograph every result you are proud of, with permission. Someone else can edit and schedule. That costs almost nothing and beats any retainer.
Buy the technical work only
What is genuinely worth paying for is narrow: campaign setup with correct radius and scheduling, the Google Business Profile done properly, and someone who can tell you what a new client actually cost. That is a few hours of specialist work a month, not a full-service retainer.
A competent freelancer covers this comfortably for a single salon. The test when hiring is simple — ask how they would target a salon in your area. If the answer does not immediately mention a tight radius and day-parting, they will spend your budget advertising to the whole country.
When an agency does make sense
Several branches with different catchments, a retail product line, a bridal division with its own seasonality, and a booking system that needs integrating — that is a coordination problem, and coordinating it badly costs more than a fee.
Below that, the honest answer is that agency economics rarely work for a single salon. A retainer that consumes most of what you would otherwise spend on reaching clients is not marketing support; it is a fixed cost with a report attached.