Let's assume $ k = 2^m $
Then to merge two lists of size n it takes $ c_1 * n $ and there are $ k/2 $ such lists thus = $ c_1 * n * (k/2) $
Then to merge two lists of size 2n it takes $ c_2* n $ and there are $ k/4 $ such lists thus = $ c_2 * n * (k/4) $
- - - - - - - -- - - - -
Then to merge two lists of size $ 2^{m-1} $ it takes $ c_{m-1}*n $ and there are $ k/2^{m-1} $ such lists thus = $ c_{m-1} * n * k/2^{m-1} $
if $ c_1 = c_2=------------=c_{m-1} $
thus total time = $ O(kn\log{k}) $