To my experience TechTrade is the best way to make money, and maybe even to win the game....
I usually invest 80% of my income on the technology research. The one´s who don´t present danger to me I sell, always the same technology to all other races in just one step of play... so they won´t sell my technologies to each others (and make my bucks...) and I repeat this race-by-race until the other races have given me all their money.
From the money I upgrade planets, buy ships etc. while they can´t do big steps without having money....
I repeat this next time I be able to speak with them again, always careful not to trade anything of the yellow-technology branch so that their trading abilities won´t rise up to mine...
The advantage of this is really big, and I´ve never played a game in which to Ai was so smart as to somehow counterattack this strategy.
The crux is that when I offer a technology the Ai will always buy, even when it´s a technology that the Ai doesn´t have use for. I think that should be changed. The Ai should try to determine if an offered technology is more useful than the money he´ll like to pay for it.... until now this determination is always "Yes", and this is his error in thinking.
The only way this won´t work is when a game is played in which I am into battle right-from-the-start with all other races involved, then their wil to invest some money will only result in a few bucks.... but in a huge galaxy size game there are usually always some non-hostile races to me...
