Get there as early as you can. I had planned it perfectly to be in Annapolis by 1 (I like exploring towns), unfortunately one mishap with my first plane made me arrive at the Academy around 5:45 pm. It didn't cause any problems, but a lot of anxiety.
So if you're coming from a very small airport like me, get there as early as possible. If you have more choices, time yourself for BWI around 3:30-4:00 pm. That gives you plenty of time.
You'll leave at 11 am Saturday. Money, that's up to you. I only brought what was needed for the shuttle and airport food, but I saw plenty of people that went to the Mid Store.
This is all assuming the schedule for your weekend is the same as the one a couple weeks ago.