Research cost is not the only factor that the AI uses in determining how much it'll charge you for a given tech. However, since it is the primary factor, generally you can narrow down the techs that take the longest by seeing what is most valuable to them; offer up your best tech and see what techs (individually) they won't trade for it.
If your diplomacy is subpar you may need to offer up multiple techs, but take care not to offer techs in the same branch (example: Impulse Drive, Warp Drive) simultaneously, as this makes them significantly less likely to make a deal. Furthermore, you want to avoid adding more than perhaps 4-6 techs on your side as an absolute maximum; the more you offer, the less each component is worth individually. This also works in the opposite direction-sometimes you can get more out of them by adding more things to the trade, as you can decrease the cost of the primary tech you want to get more than the secondary tech's cost.
Additionally, having about 25-50 more diplomacy than twice your target's diplomacy gets you the best deals-anything much above that and they simply won't pay more/let you offer less. The one exception to this is the Galactic Bazaar, and that acts as a 50% modifier separate from this "diplomacy cap".
Finally, when you're trading techs to the AI, start with the most modern branch of a tree first and work your way backwards-one at a time, if possible. So sell them Warp Drive, then Impulse III, then Impulse II, then Impulse, and finally Ion. This nets you something on the order of perhaps 33% more cash/value, from memory.
