From what I've discovered when trying to play catch up on higher levels of difficulty (Regent and above), is that Military civs tend not to research 'Medicine' and 'Sanitation'. So you can often milk them for as much gold per turn as possible, with that one. I was got as much as 110 gold per turn for this particular tech advance.
I suppose it depends on the civ in question, and what their cultural tendencies are (forgotten word for this), as in, military, religious, scientific etc. The point being, some civs tend to go for either scientific, military or religious tech advances that coinside with their 'cultural tendencies'.
The only way to tell, is to keep trying to trade with em, and see if they want any of your techs - unfortunately this will probably happen in the industrial ages, unless you're really lucky.