I think it depends on your location. If you're in Europe, the Phils are the cheapest, then Maple, then Eagle. Vice versa, in the US, order Phil/Maple probably swapped, and ASE has a high enough premium to stay the highest.
What I do see here in Europe is that Maples are quite alot as 'special prices'. I get the impression that if people sell silver back to dealer, that the Maples go first. Maybe due to the milk spot story. The first bullion I ever sold were Maples, but it was also due to having a lose tube while selling something else would mean an uncomplete box. Nevertheless, if I ever gonna sell a box, it will be the Maple one. Milk spots bah.