I always keep about 1,000 gold on my bank alt for rainy days. You never know when you'll need it.
Last night was a good example. I managed to squeeze in an hour's play time and after killing King Magni we were able to put together a guild Vault 10. Well I say guild but we still needed 3 members from an allied guild.
I was MT, and while keeping an eye on Omen was pushing upwards of 4200 TPS when he wasn't picking me up and tossing me across the room.
After killing him, he was nice enough to drop a new pair of gloves for me, specifically the tanking variety. Too bad my other Tier piece is the DPS chest. In any event, these had 28 less defense rating than the daunting gauntlets I had been wearing.
So I needed to regem at lot to get back to the crit cap. I also had to enchant these. I went with armsman for the extra threat and parry. All told I spent about 300g.
This is where the emergency fund comes in. I'm not a farmer. Its not in my play schedule to grind for hours or fly around and herb every zone. Therefore, when I'm less than 1,000g I will focus on getting back to that level as quickly as possible. Once there I will stop so I can go back to doing what I enjoy most. Running heroics and raids.
Keep a stack of gold on hand, you'll never know when you'll need it.