Well, costs for enemies are simple: add their stats and divide by two :D. Threat is totally different, though I read something about Spiders in the Core set having a lower threat than their stats indicate, maybe there is a formula there.
The bigger problem is, that enemies can do all at once: "quest" as long as they are in the staging area, attack at the beginning of the encounter phase and then defend against player attacks. The next thing is, that player characters for the most part are specialized in one task. A Galadriel's Handmaiden for instance would hinder your questing, but play no role in combat: just engage and "tank" her, she does not deal any damage anyway. What about Beregond? He has no willpower, so he is useless in staging and his single point of attack won't do much damage, so why would you bother killing him? You need 5 attack for just a single wound on him. Now compare his stats to a Cave-troll. Same defense but all other stats are higher by 3 to 5 points, so it would need a threatcost of 21 (or cost 10+ resources). Not to mention a built in Firefoot.
As nice as the idea is to turn the tables, enemies and characters are built asymmetrically for good reason, so I fear even with an algorithm for swapping threat/resource cost and engagement cost, this would not work in the end.
When thinking about this I now see, why the digital game is, as it is. Gameplay is far more symetrical between good and evil, characters and enemies can guard their comrades, cards from Sauron cost resources and so on. Maybe FFG at one point allows monster play in the digital game, but for the LCG I see no way.