The cost of putting the hob on the island depends on how easy it is to get power there for it, and on the details of the extractor you want. A recirculating overhead extractor will be cheap.
An overhead extractor ducted to extract outside may be reasonably cheap (e.g. if the ducting can easily be installed between joists while the ceiling's down for other reasons, or if the ducting can be boxed in alongside a new beam or just go straight out through the roof of a new single storey extension).
A downdraft extractor ducted externally could be very expensive, especially if you need to dig up an existing floor slab to place ducting under it, then reinstate the slab afterwards.