A few factors make for ISC operations being expensive:
1. For tourists it is a summer operation only, during the winter the operators only clientele might be the islanders on their day trips, or similar, to the mainland and might only justify one or two flight operations per day, up to six days per week ... And there are/will be two dedicated operators competing for this poor winter trade.
2. Even during the summer midweek trade is lacking and ISC dedicated aircraft, crews, ground staff, indeed LEQ & PZE airfields, won't be fully utilised. In the days when Brymon operated ISC routes they would operate an additional DHC6 just for the summer whilst during the summer midweeks they might only operate one ISC rotation per day whilst on Sunday's ISC was closed except if they had been fogged out on the Saturday, but, for Brymon, no problem because their DHC6's would also operate such routes as LGW, EXT, JER, GCI, ORK, MXN, BES, BHX & EMA to occupy the remainder of their days.
3. About the only busy, perhaps profitable, day would be Saturdays, Brymon would have 2 or more DHC6's dedicated all day to ISC with routes from/to PLH, NQY, EXT and BRS but even then ISC is subject to sea fog and with so little land available for the sun to heat to lift the fog the sea approaches would often remain fogged out all day ... It's a tad difficult to achieve an RVR (Runway Visual Range) in excess of 1,500m with a runway length of only 694m!
So it's not as if another BN2/DH6 operator could even come in just for summer Saturdays thus it's likely to remain the closed shop with associated fares that it is indefinitely.