"The actual launch cost is in the hundreds of millions"
Not quite.
I work for a commercial launch provider, and the cost to put a large satellite (DirecTV or Intelsat) into GTO is $95~$110 million, inclusive.
You ship it to us, and we'll get it into space at the correct injection point.
Since we're always looking for new revenue streams, I specifically brought up the possibility of releasing cubesats or "other" secondary payloads before primary spacecraft separation, or after if better, as I know we have the capability to support it.
Top management replied (and knowing these guys personally I have no reason to doubt them) that they looked into it seriously, BUT.....
Since we don't build the "Payload Accommodation" (launch adapter, satellite support structure, fairing, avionics, ordinance, etc), we're at the mercy of the contractor who does build it to add in the capability of multiple satellite deployments, and the cost figure they came back with made it "economically infeasible to offer a secondary payload deployment service".
So while you may see launch providers that say they have the capability to drop off secondary payloads on their way to GTO, once you start talking "How much?", you'll find the game changes rapidly.
73, Jim KQ6EA