Shoreham is really busy. A lot of UK GA moans and whinges all the time especially on the forums but enough people seem to disagree with the above mentioned views. If Shoreham dropped its prices it would just attract more of the five quid pilots who they don't need anyway because on the nice days these people fly, airports like Shoreham are really busy already.
It's a free market and they can charge what they like as long as the traffic will bear it.
That said it's about 10 quid more than Cambridge, Biggin or Prestwick, three airports I use frequently and, AIUI, I don't think the facilities at Shoreham are really comparable to any of those. BTW, what's the fuel price like at Shoreham?