With tech trades occuring instantaneously, the mechanics of the diplomacy system give rise to a powerful dominant strategy; namely, when a tech is held by two or more players, either player has a strong incentive to be the one to sell the tech to the rest of the players. There are a few complications to this simple picture (like war), but for the most part techs "should" be traded aggressively by all players... The problem is the AI doesn't pick up on this.
The resulting weakness of the AI in this area has been a popular topic. Stardock has tried to "tighten up" up the trading, which may or may not have improved it, but hasn't fixed it. The problem is beyond something that can be fixed by multiplying the value the AI places on a tech by some constant; the problem is the AI doesn't understand the tech game.
Have you ever been prompted by an AI player looking to sell a tech for money? I haven't, although in my first few games an AI player would occaisionally offer a one-for-one tech trade. I recall Frogboy writing that the AI initiates tech trades in the same way for human and computer opponents, so this right here is one of the problems: the AI doesn't have the same options for selling techs available to it as are available to the player. No tinkering with multipliers can fix this; in fact, increasing the cost ABOVE the optimal value will result in loss for the AI just like undervaluing techs will, although the loss here is from lost opportunity to participate actively in the tech market (consider this: whenever two players trade, everyone else loses--with some minor exceptions for alliances, etc).
Tech trading off plays much better, in my humble opinion. I'm afraid that a true, solid fix of the problem will require signicant reworking of the tech trading mechanics (not just AI, because if the AI played this set of rules well it would require the player to intensively micromanage tech trades no matter what). I haven't seen any indication that a thorough rehauling of the mechanics is on Stardock's table (and well beyond what's moddable so far), so I dunno, I'm pretty pessimistic about it right now. But tech trading off is ok.