This is a general comment about what being a club member is about. Whilst it's been sparked by points raised specifically about caravanning, it applies to membership of all clubs,
There would be little point in joining a club of any sort of it did not offer you some sort of benefit, so you would be rightly miffed if a clubs facilities were open to non members at the same rate. But access to certain facilities may not be the only membership privilege, so you have to draw up your own cost vs benefits balance sheet.
But from a non members perspective, you shouldn't be surprised that access to some club owned or controlled facilities might cost you more, or are simply not open to you, because your not a member.
Clubs are invariably also businesses, and as such for a facility to remain accessible, it still has to cover it's costs, consiquently even club members will still have to pay enough to cover the running costs, which might bring the cost closer to general commercial costs. What you are paying for is relative exclusivity. To help maintain that exclusivity, the facility owners will charge more to non members, to discourage them.
One of the consequences of trying to make a facility "exclusive" is it limits the potential number of users, and that means certain running costs will be similar to commercial operations, and those costs have to be covered by fewer customers which can raise prices.
Some club opportunities do offer some savings, but it's better to think of benefit of being a member of a club gives you more exclusive access to a wider range of facilities rather than necessarily always being a cost saving.