Visitor

You can pay at the gate normally. If you are a member then I believe it counts for two adults. "Adult membership
1-2 adults" but the wording isn;t clear.

Members of the RSPB get free admission to RSPB reserves with an admission charge. If there is a car park charge, but no individual admission charge. Some RSPB reserves only have a car park charge for non-members, but RSPB members get free parking. There are also some RSPB reserves with no car park charge or admission charge for non-members. I also been reading that some RSPB reserves with an admission charge for non-members are increasing  cost of admission.

Depends where you go. There are also some very good non-RSPB places, esp at this time of year. Roughly where in UK will you be based?