How about this?
Prices = Dict([
(1,2) => 10
(1,8) => 7
(2,2) => 9
(2,5) => 8
(2,7) =>4
(3,1) => 9
(3,6) => 5
])
Orders = [(1,2,1) , (1,2,2), (1,8,1), (1,8,2), (2,7,1), (2,5,1), (3,1,1), (3,1,7)]
Price_Orders = Dict([(a,b,c) => Prices[(a,b)] for (a,b,c) in Orders])