Visiting the Isle of Wight

I was trying to book a return ferry journey from the UK mainland going to Isle of wight (Portsmouth Gunwharf to Fishbourne) at the wightlink's website when I came across a peculiar problem. I selected one of the days out ticket combined with the ferry travel and the cost of the ticket came up as £58 for 4 adults and a car. As soon as I added a 5th adult, the cost rocketed to over £2000 (I would rather buy a second hand car and book 2 cars one with 4 adults and one with 1 :)

Here's how it looked: