You're right. You could spend a whole day on HMC and still have lots more to discover. It's a shame that most of the BH trips don't allow much more than an hour there. Seeing the booby birds is certainly fun, but a slow amble along the south shore (the one facing the reef) with its multitude of fossils - you can't move without treading on them - is fascinating.

On the park fee, only US$10 is for landing on HMC. The other US$30 is for visiting the BH, so if you haven't done that you needn't pay.