There is no charge to self park in the Boardwalk lot.
The lot at the Boardwalk is intended for guests of the BWI/BWV and for guests that are visiting the Boardwalk area.
At times, it is very difficult to find a space in the Boardwalk lot. That is a frequent complaint of the BWI/BWV Hotel guests. The lot is not big enough to handle everyone who wants to park there, especially in busier seasons. (I myself had quite a bit of trouble finding a spot in the late afternoon when I stayed at BWV the first week of December last year). The CMs at the entrance shack discourage people from parking there to go to EPCOT (actually turn them away if they say they are going to EPCOT), especially in the mornings when there is nothing much happening on the Boardwalk. That said, many people do park there and go to EPCOT. After Illuminations, you will see lots of people leaving.