A presenter is the face and the timing. On camera they carry the piece; at an event they hold the room together and make sure the thing that was supposed to happen at seven happens at seven.
Day and half-day rates, with rehearsal or prep sometimes quoted separately. For events the rate usually assumes a script or run of show exists — if it does not, someone has to write it, and that is a separate job.
Watch footage of them working rather than a showreel where they are cut every four seconds. What you are buying is what happens in the unedited minute when something goes wrong.
You say what you are trying to achieve rather than what to buy. Lucy shapes it into a brief, prices it against what people here actually charge, and shows you the creators who can do it. You agree a figure before anything starts. The money is held until you approve the work, and Lucy takes 15% of the creator’s fee — never a fee from you, and never anything on approved expenses.