Just reread this thread - I've been using Qitai's trick regularly when I need to keep the AI happy - that is, until I'm ready to knife them...
I really don't understand Ribannah's contention that this is supposed to be an exploit and hinders noobs from learning good strategy..
Good diplomacy is obviously a must at higher levels - you can't possibly be at war with everyone all the time at Emperor or higher and have a prayer of winning. Thus, it's an integral part of strategy!
Giving a tech for less than the market value
should result in an attitude bonus from the AI. Since the programming doesn't give it, Qitai's trick just rectifies an imbalance, doesn't create a new one, IMO.
Another matter is the peaking of the attitude bonus at 100 gp regardless of era - this is an obvious bug that should be rectified, it's totally illogical!
Such a limit shouldn't be hardcoded but variable by era, or maybe even by turn - or additionally by difficulty level.
Personally, I use this gambit only in the ancient age, where 100 gp is still a lot of money. From the Middle Ages on I'm usually strong enough I don't need it anyway, so I don't feel I'm exploiting the 100gp-bug.