The fare is made up of two parts: The rail fare to Portsmouth Harbour plus an add-on for the ferry (or Hovercraft) and Island Line journey.
As Hedge End to Portsmouth Harbour is only 17.75 miles, there's no period return for that leg. As such, the price for that leg is just double the Single fare of £11.10 (i.e. £22.20) without taking the possibility of using an Evening or Sunday Out ticket. Deducting that from the through fare reveals the add-on to be £50.40.
If you buy the tickets separately, however, Hedge End - Portsmouth Harbour would be either £14.60, £18.40 or £22.20 (depending on whether you were travelling at a time the Evening/Sunday Out tickets were valid or not), a Return on the ferry would be £39.00, and Ryde Pier Head - Shanklin would be £9.40, £10.40 or £11.40 (again depending on whether you were travelling at a time the Evening/Sunday Out tickets were valid or not.) So you'd be looking at a total of £63.00, £67.80 or £72.60.
The best way to reduce the price would be to buy a Two Together Railcard. That would cost £30, but brings the through fare down to £48.60 per person, so £97.20 for the two of you, making a total of £127.20 including the cost of the Railcard. As a result, it's paid for itself with just that one trip!