This problem is a modification of 72. Edit Distance. The idea is very similar, suppose that we have a function f(x, y) which could return the minimum distance for word x and y. Then we can have this recursive formula:
if x[-1] != y[-1]:
f(x, y) = min( f(x[:-1], y)+ord(x[-1]), f(x, y[:-1])+ord(y[-1]) )
#delete x[-1], delete y[-1]
else:
f(x, y) = f(x[:-1], y[:-1])