Merchants cost 550, and their upkeep is 0.

So far playing as England (turn 88) I have around 6 merchants, all paired up in twos and sitting on the same resource in the same provinces. I have two in western france trading with wine, two in north east france trading with silk, and another two in HRE trading with iron. With each two im making about 60 - 90 florins a turn, depending on their level of experience.

I did have a guild in one of my cities that gave a bonus for two of my merchants (they both started on level four), I think it was the Theopalian (sp?) guild.

The best tradable goods ive found around England so far, is wine, tin, and silk.