Caroline,
We are driving to Turkey in October, we are also combining it with 3 nights on a ferry (Ancona - halfway down eastern Italy) to Cesme (near Izmir).
In round figures going via Germany to ferry is 1500 miles from Calais, we are stopping over for 2 nights. From Cesme, it's about a 6 hour drive south.
We want the single ferry bits, plus we did not want to drive through some areas.
Naturally costs will vary with your needs and transport.
I trust this is helpful.