Even if you accept the tremendous hassle of parking in BWI's long-term lots, they charge $7 per day with 7th day free, so you'll spend at least $70, and perhaps $77 if you arrive later in the day than you depart.
If you're willing to deal with the shuttle bus hassle anyway, consider taking the train, especially if you have weekday arrival and departures that allow you to use MARC. Taking the shuttle from BWI rail station is no more inconvenient than taking the shuttle from the parking lot, and you can get train fare for as little as $5 each way.
Also, be aware that the lower level of the terminal is a *huge* mess right now due to construction. If you do use one of the parking options, on your return I recommend that one member of your party retrieve the car while the others get the luggage. Then have them drag the luggage up to the ticketing level, and pick them up there. Just flew from there at Christmas, and this saved us a lot of aggravation.