Perhaps the answer lies somewhere between bingoboy & bose-x.
Keep the visitor landing fees at a level where the amount of traffic is at a maximum consistent with not attracting so many visitors that it adversely affects the home-based users.
That way the home-based users benefit from the visitors increasing the turnover on based facilities like fuel, food, maintenance etc.
If you reduce the number of visitors by increasing landing fees you not only risk hitting your income from landing fees, you also reduce the turnover of the based businesses, and ultimately the rent they are willing to pay you and the level of on-airfield services for the based pilots.
Mike