good question mate
I checked the "statistics" in the player account management screen recently. There, it shows how much xp,gold etc your character received over the last weeks. It needs some notes from yourself though, to know which day you went in with full gear, light gear etc.
Its just a guess, but I think the more single items you wear the more expensive it gets as the potential for repairs increases.
For example: a shield and a spear cost less upkeep than a shield, a spear AND a an additional 1h weapon etc etc.