The orchestra is going to want to pay one fee for the harp rental and all other related expenses, mainly transportation. They will want to show up at the concert hall and have the harp sitting there ready to go. And when the concert is over, they will just leave and have you deal with picking it up.
The fee depends on how much transportation is involved. Is this a one day rental? Rehearsal in the afternoon and concert in the evening? You will probably have to attend the concert so that the second it is over, you are there to remove the harp. I would say, depending on how much time this involves for you and how much driving you have to do, that the fee should be a minimum of $400.
I used to rent a harp to the traveling Frank Sinatra show when he came to the Boston area. He traveled with a 35 piece orchestra, including a harpist. But his manager would rent a harp in each location. So I had to deliver the harp to the orchestra pit by 4:30. The orchestra would arrive soon after and there was a sound check from 5 to about 6:30, then a dinner break for the orchestra, then the show at 8. There wasn’t enough time for me to go home after delivering the harp and then come back around 10 to pick up the instrument. In addition, had I gone home, I would have been returning as 15,000 people were leaving! So they always gave me a back stage pass and I could just hang out, listen to the sound check and the concert, have dinner with the orchestra, and stand backstage as Frank and his guest artist walked out at the end of the concert. This was all 25 years ago, and at that time I charged something like $200, possibly $250. I can’t remember exactly.