Prices vary by time of day, together with the number of passengers in your car, etc.
However, looking at some sample dates in August (and assuming that you don't want to travel around either dawn or midnight), you're looking at a typical return fare of around £208 on the Fishbourne to Portsmouth route (with Wightlink) but perhaps only £197 on the Southampton to East Cowes route (with Red Funnel). The Lymington to Yarmouth route (Wightlink) is coming up far cheaper though, at around £104.00.
The other routes to the island only accept foot passengers.
NB: Some economy fares are already close to being sold out. Book early!
http://www.wightlink.co.uk/
https://www.redfunnel.co.uk/en/