You need to add in the Passenger Facility Charge as well. $4.50/pp
No, we don't need to add the PFC. PFCs are charged on a per-passenger basis (not a flat fee like landing fees) and are billed to passengers via ticket sales. They're a separate line item on tickets.
Additionally, the existence of a PFC at LGA will have no impact on WN. Southwest has thrived for many years at airports imposing PFCs, including BWI, BUR, MDW, LAX, LAS, PHX, SJC and OAK, among many others (all of the above are $4.50 but a few of WN's airports are only $3.00). That list above includes most of WN's busiest stations. In fact, nearly every aiport served by WN imposes PFCs except for DAL. Even ISP imposes a $4.50 PFC.
Here's a list prepared by WN showing PFCs imposed by airport:
http://www.southwest.com/travel_center/pfc.html
Bottom line: PFCs are a constant and thus, we don't need to "add them in" when discussing WN's advantages/disadvantages.