It seems to me that Romans do in fact come out ahead. In my mind, I think that Romans typically build their infrastructure up better than a typical Teuton does, so when you do in fact wipe out a Teuton army, it'll take them much longer to get back on their feet. You can keep raiding and reraiding and catching their troops with just 3-5 Roman soldiers and should be able to wipe them out each time. Teutons just don't have the resource production (again, I could be viewing it wrong, I don't know) to go head to head consistently at this point in the game.