The most valuable resource seems to be...

[Gingivitis]

Map Information.

Factions would rather just hand me florins than part with the priceless info about their 4 regions on the other side of the world from me. It could be ages since the last time we traded maps and I may have 1/2 the map in my pocket, but darned if they want any piece of that. Any hints on how to pry this goldmine of knowledge from them?

[Carl]

If they don't want to give it to you it probably means their map hasn't changed since last time.

[vonsch]

I haven't found that factions are very avid map traders. So you don't get much more than their own map, and that doesn't change fast. That's a big help to the actively diplomatic player, who can swap the first couple, then start making a nice profit selling them. Or boost relations by being generous.

But, yeah, in my experience if they won't swap maps, you already have it.

[HoreTore]

I nelieve that when you trade maps, you don't get all of his map info, you only get his provinces and cities, plus the LOS of his units(like diplomats) who are in other peoples territory....

Like for example, when you trade maps with the moors and they have a stack halfway to timbuktu, you only see a blob where the stack is, not the parts they discovered going there...

[TevashSzat]

The thing is when you trade map info, you give everything you know about the world to the other faction even knowledge of foreign lands so far into the game, an ai faction can easily have the map information for the whole world and if you are approaching them with only a 1 - 2k offer, they will almost always decline it

[Gingivitis]

Yeah, still seems dopey they'd rather just give me 300 or so florins for nothing instead.