It is the price for the Valet Parking, and it is a fairly standard fare for a 5 star hotel. I paid 60 USD per 24h at Hilton Boston Faneuil.
There are parking garage (covered and with CCTV) around the hotel, I remembered seeing two less than 3 blocks away. You will pay less there (probably between 30 and 50 USD). Parking in downtown cities in the US is always expensive. Short term parking is usually 15-20 USD.