I've worked out what's going on with these car rental prices. When I enter a rental for exactly 1 week some listings are back down to the normally expected prices and about a third or even less of the prices for 11 days. Ridiculously this also means that 2 weeks can be far cheaper than 11 days since it also must use the 1 week rate whereas 11 days must be using a much higher daily rate!

.That said looks like some providers are sticking only to a daily rate.
I am seeing the same thing on several car rental search sites. I have had 11 night rentals in Ibiza before and don't remember this however that was years ago so maybe this pricing policy has been around a while. The past 2 years I went were 7 night trips so I didn't notice this issue.
Surely there must be a rental provider out there that can recognise 11 night duration at the longer term weekly rate so will keep searching and contact them directly if necessary.
Also seems late night/early hours morning pickup/drop off times can mess with the prices. I suppose that will likely be 'out of hours' charges however it isn't transparent in pointing that out as the reason for a 1 week rental being say £30-40 higher than a 1pm pick up/drop off.
So perhaps the most sensible approach value-wise will be to go for a 7 day rental during office hours and I can go without a car for a couple of days at the start and end of the trip


. Just a bit annoying though as I always feel leaving your luggage in the car boot on the final day when you have a night flight is less risk than leaving it in the hotel luggage store room.