Take out your Scrabble set, note down the scores for each letter, total the scores for a word, hey presto you have your algorithm. Not sure it entirely satisfies your requirements, but it might point you in a useful direction. You might, for instance, want to assign scores not only to individual letters but also to di- and tri-grams.
I'm not aware of any existing source of the information you need, perhaps you could come up with your own letter scores by examining the keyboard and assigning higher scores to the more difficult letters: so 1 for 'a', 8 for 'q', 2 for 'm', and so on.
EDIT: I seem to have confused people more than I usually do when I reply on SO. Here's the barebones of my proposal:
a) List all trigrams and digrams which occur in English (or your language). To each of them assign a difficulty-of-typing score. Do the same for individual letters (after all a 4 letter word might be composed of a trigram and a letter rather than two digrams).
b) Score the difficulty of typing a word as the sum of the difficulty of typing its components.
As for the difficulty scores, I haven't a clue, but you could start from 1 for a letter on the home keys on a keyboard, 2 for a letter which uses the index fingers but is not a home key, 3 for a letter which uses the 2nd or 3rd fingers on your hand, and so on. Then for digrams, score low for easy letters on left and right (or right and left) in sequence, high for difficult letters on one hand in sequence (eg qz, though that's perhaps not valid for English). And on you go.