On our fleet all the aircraft are now fitted with winglets. Reckon they managed to get them for next to nothing, so already have increased the value of the fleet and the resale price tag.
Quite alot of our trips are for example 4 sectors, made up of three hours there and back followed by a short sector of one hour there and back. So it would be very difficult to juggle aircraft if it was mixed fleet of non and winglet ac.
On the longer trips the fuel burn is often half a ton less than planned, with maybe only 0.1 of a ton saved on the shorter trips.