numbers might be off slightly, but this is basically how it works:
every tick of gold/xp that you get there is a 4% chance for each of your unbroken items to break, costing 7% of it's base cost in upkeep. Things can't break more than once, but if you have two of the same item they both can break (like for archer quivers). Items that take up more than one slot have to 'pass' 2 (or 3) of those 4% chances per tick to avoid being broken, so basically the more slots something is the more likely it will break. Oh yeah and arrows have an increased break chance, don't know what that is exactly.
I hear that the more wpf you have for a weapon the less likely it will break. Don't know about exact numbers though.
So if you have an item that costs 100 gold, repairing it will cost 7 gold. 1000 gold, 70 gold etc.
So if your current equipment loadout costs 1500 gold, the MOST you'll pay at the end of a round (assuming everything breaks) would be 105 gold.
With the equipment you have now you should be gaining gold at a steady rate. Being on the winning team and getting higher xp/gold multipliers per tick will allow you to maintain higher levels of gear without dipping too much in your savings. I recommend having a backup supply of gold before you burn it all on a new piece of armor/weapon so you can absorb the unlucky repair times.