In reply to ChrisJD:
Mostly because it's even more expensive! We recently avoided it on May BH for that reason. £72 for two people for a weekend's camping is a touch steep.
On the other hand, they close it once it gets full, compared to other campsites that just keep packing them in. The campsite we eventually stayed at got disgustingly full, to the point where we couldn't actually park on site anymore. Not ideal for somewhere with only three toilets in total.
If OP is planning on a bank holiday weekend they might do well to rough it a bit, and go somewhere like Nant Peris or the one by Llynnau Mymbyr as a compromise. Both were pretty empty when we drove past on BH saturday.