It’s a rare boon: of the 400 or so holidays we spot-checked, we only came across 26 instances where the longer stay was cheaper. However, the savings can prove to be substantial.
For instance, a Tui holiday to Fuerteventura was more than £1,100 cheaper if you tagged on an extra night at the same all-inclusive hotel (with the same departure date). That means a couple could save almost £600 per person by extending their beach time by a day – with all their food and drink thrown in – just £1 in one example.
A seven, 10-day or two-week holiday is more typical, but it’s worth bucking the trend to see if this works for you. Just make sure you compare like for like – and the 11-night holiday isn’t cheaper because of a room downgrade or lower-board basis.
source https://www.which.co.uk/news/article/longer-holidays-cheaper-aWsZB0n3a3dO