http://wiki.c-rpg.net/index.php?title=UpkeepSay you played a round that lasted 2 minutes with the Nodachi. There is an 8% chance that you will have to repair it when the round ends. The system will then take from you around 1000 gold (7% of the total cost of the Nodachi, which is 13,766). The 4% chance applies to every item you have equipped. More equipped items = More repair costs.
Also, weapon proficiency in the same category as the equipped weapon will lower the chances of repair slightly. So with the 150 proficiency in Two-Handed, instead of 4% chance of repairs, it's now around 3%. I'm not sure of the exact percentage effect proficiency has though.