Private tours make most sense where the group vehicle is the compromise: a private van to Butchart and the Malahat lets you set the pace, a private Zodiac from Victoria goes where the whales are without forty other passengers, and a private charter around the Gulf Islands is a different day from a scheduled cruise.
Prices are usually per vehicle or per boat, so a family of four often pays little more per person than a shared tour, and much less than they expect. Confirm the maximum group size the price covers and what is included — admissions, lunch, fuel — in writing.