Not sure how much help this will be but...
Looked at some Honda's and an Alphard in Gosport not long before we bought our Vellfire. The salesman did say that they regularly replace head units on Honda's (and I think Toyota's too) for £500 but I'm not sure if that includes connecting up to existing monitors.
Have a google at japanautos.co.uk and maybe give the guy a call and see how wrong my memory is?