Devu and Locks

Devu likes to play with a lock of N dials. Each dial rotates from numbers 0 to 9 clockwise (i.e. 0 to 1, 2 to 3 and 8 to 9). You can not rotate from 9 to 0. Initially all the dials of the lock are set to 0. From the current lock, Devu can move any dial to at most 10 different positions (i.e. 0 to 9), so there are total 10N different locks possible. Let us say for a lock we define cost of c