I've always been given a free courtesy car (by BMW and MINI), whether for the day, or overnight. Smart/Mercedes always offered me free ones, too... I had an A-Class for 8 weeks once and they didn't ask for a penny!
Ford used to charge £10/day (if you were lucky enough to get a courtesy car without an 8 week wait), and would usually waive some - or all! - of the charge if your car was in for warranty work.
To get comprehensive cover for another vehicle on my own policy, there's an admin fee, plus a recurring weekly charge, so I've never found that to be cost effective anyway...
I'd be angling for a free car from MINI... although I guess individual dealers will have their own insurance policies, with different terms, so it's their call at the end of the day.
