Well, Trump said there were a million people at his inauguration (I know, this was an absurd bald face lie). I don’t know how much it costs to rent, set up and take down that many chairs, but I’m going to guess it was more than three fifty.
(I agree with your overall point though. There were speaker towers arranged at intervals down the mall and plenty of crowd control involved in getting people to and from the event in the first place, none of which had to have been cheap.)