Part of my day trip around the main island of Tonga took in this sight, and with no or low expectations of the sight from the guidebook, I was captivated by this wondrous natural sight.
Serious Tip: Get down and close to the rock edge, to see the full coast of blowholes, for the full effect.
Time Spent: It was about a 45 minute drive from Nuku'Alofa, through villages, to get there. We stayed for about 30 minutes, but definitely could have stayed longer.
Cost: Part of my day tour from Fafa Island, so difficult to pin point how much this was for me. But I am sure you can negotiate a driver/taxi from town, to bring you out to see these.
Quirky Tip: Going along to see these with such low expectations made this natural phenomenon even more magical, because I had no idea what we were seeing and why! That's what makes travelling so amazing!