The three are not price variants of the same thing. They solve different problems, and the question to ask is not "which is cheaper" but "how predictable is my work".
What each one actually delivers
A freelancer supplies hours in one craft. You buy a pair of hands with a speciality, usually available fast and with no notice period. What you do not get: a second opinion, a stand-in when they fall ill, and someone who pushes back.
An employee supplies continuity. After six months they know your product, your colleagues and your sensitivities, and that is exactly what an outsider never catches up on. You pay for it in fixed costs and in the duty to give that person work and a path.
An agency supplies a team. Strategy, creative, production and project management sit in one contract, and the whole thing can scale up and down. You pay for that coordination, even when you only needed one of the four.
The question that decides it
How predictable is your work?
- The same every week and centred on your product: hire.
- Irregular, a single craft, you keep control: take a freelancer.
- Irregular, several crafts at once, and no time to stitch it together yourself: take an agency.
The mistake made most often
Hiring an agency for work that returns every week and no longer contains any thinking. That is the most expensive way to buy execution, and it is not a good assignment for the agency either: the people you get are the ones being stretched least.
The reverse mistake is just as common: asking a freelancer for a brand positioning that five parties will then have to execute. You are buying advice without the power to push it through.
In practice the three often run side by side, and that is fine as long as someone holds the reins. Who holds them and how much belongs in house is exactly what IKAg maps out under Marketing organization. See what you do yourself and what you outsource and what an in-house marketer costs compared to an agency.