What determines pickpocket success chance?

Specifically for gold there is an unknown formula for maximum gold value that can be pick-pocketed based on the level of the character's Pickpocket skill and which perks they have. This can result in situations where even a small change in gold value can result in huge swings in percentage chance of success.

2000 gold is a lot of gold, it's going to be pretty hard to steal it back unless...

One way to completely 100% guarantee a successful pickpocket attempt is to paralyze the target first. As soon as the victim begins to get back up, try to pickpocket them and you will be able to take anything no matter the percentage chance displayed.

It is good to note that if you pickpocket this way your level will not increase.

Source: http://elderscrolls.wikia.com/wiki/Pickpocket

Though on another website they seem to have the formulas for chance to steal on stealing items and gold

Base_chance_formula = ( ActorSkillBase + Player_skill ) × ActorSkillMult + ( TargetSkillBase + NPC_skill ) × TargetSkillMult - Detected

Stealing Gold or Single Item: ( Base_chance - Gold_amount / 10 - 4 × Item_weight + Sneak_bonus ) × Effects_mult + Light_Fingers + Night_Thief + (Cutpurse if Gold)

Source: http://en.uesp.net/wiki/Skyrim:Pickpocket