The AI is inconsistent about the value of obsolete techs. On the one hand, it complains loudly if you try to trade it both Laser II and Laser III, and it will throw in Laser II for free if you trade for Laser III. On the other hand, it will gladly pay for Laser III, then Laser II, then Laser, then Beam Weapons. Even on Masochistic you can get 700 bc out of the laser techs this way, or give them away once a turn, or trade up all their cheap technologies. It's an exploit and a waste of time, but it's very tempting and the computer talks like it should know better. I'd have it say, "Sorry, I already have Plasma Weapons. I don't need (Laser III)."