In selling map info during early game I got myself a good procedure:

First I check the financial status of the faction I'm going to deal with by looking at the financial graph, assesing his diplomatic situation (lots of enemies might mean he'd rather be building up his armies than buying maps), assesing his settlements, etc.

Then based on my appraisal I charge this faction with 500 denarii if they made about 4K - 5K denarii the last turn (based on the financial graph), 700 if they made more, 900 or higher if they're really rich. Sometimes I do offer 400 denarii or lower if I really need the money and the faction I'm dealing with just made 2K - 4K the last turn.

I do check out the financial situations of the other factions regularly, 'cause it affects all kinds of negotiations. I've had these experiences so far:
1.Offering map info with a very reasonable price 400K or less and ending up with the Tactless trait for your diplomat. I just failed to check on the financial graph and was actually offering the other faction with something they just could'nt afford.
2.Once Gaul came up to me and asked for a ceasefire. I named my price and they just could'nt agree and my offer came to as low as 100 denarii for 5 turns and they just would'nt accept. And when my turn came up I just realized they were so financially ko'ed and they were engaged in a war with us (Julii), the Britons and Spain. I offered the poor bastards peace for map info instead and established trade rights so we could make each other some money and they could survive a little longer as my buffer against the other factions.