My room (and the cheapest you can get in Aron hall (basically on a lower floor, with 4 or 6 rooms)) is something like 573 dollars a month. The "top tier" rooms (upper floor, duplex, with a balcony) can go up to about 700 dollars a month. There's a range of rooms in the middle that go for 600 some dollars in the middle floors and the cheaper duplexes. They must have taken an average of all the available rents in Aron Hall to make that budget.
Once you start living in Aron, you're responsible for the whole year. You can't terminate for MS1 summer (at least going through a complicated process and you would probably have to pay a fee to get reinstated into Aron later anyway). Most of us just found somebody who was here doing research or one of the HuMeds who was coming in for their summer program to sublet for the whole period. It's really easy to find a subletter, don't worry. (note you can't charge them for more than what the room costs) The same goes for if you ever have to leave Aron for any period of time (like away rotation).
Also re: budget, it always is kind of inflated. I spend ~$100 on books and copying, and you don't need to buy any of the equipment they list on that budget. There, I just knocked off almost $2k from your budget 🙂