As of patch 1.0.4 the maximum gold find is now capped at 300%. The only exception is Nephalem Valor which can bring the total Gold Find to 375%.

Pre patch information:

According to datamining sites (example) the relevant item affixes are:

• Amulet: Prosperous: Chance to Find Gold Increases by 41-50%
• Ring/Armor/Offhand: Prosperous: Chance to Find Gold Increases by 21-25%

So, here is a try at the most possible gold find.

• Amulet slot: 50%
• All other slots except weapon: 11 * 25% = 275% (head, shoulders, torso, wrists, hands, waist, legs, feet, 2x rings, offhand)
• Helmet gem slot: 31% (Radiant Star Emerald)
• Follower amulet slot: 50% * 20% follower bonus ratio = 10%
• Follower ring and offhand slots: 3 * 25% * 20% follower bonus ratio = 15%
• Nephalem Valor: 75%
• Shrine of Fortune: 25%
• Demon Hunter Ferret Companion: 10%

Total: 491%

There may be some legendaries with increased gold find, so more than this may be possible. However this is not at all realistic, because 1) it would take ages to find perfect "Prosperous" items for all these slots, and 2) your gear would likely be decidedly suboptimal for your class if all you sought was Prosperous items.

Overall, to maximize profit, I think you would do better optimizing for your class's DPS stats, and adding in as much gold find and magic find as you can without hurting your kill rate or survivability.

Normal difficulty

Act 1

1. The Fallen Star: 1-6
2. The Legacy of Cain: 1-9
3. A Shattered Crown: 2-10
4. Reign of the Black King: 4-13
5. Sword of the Stranger: 5-13
6. The Broken Blade: 6-14
7. The Doom in Wortham: 8-16
8. Trailing the Coven: 8-16
9. The Imprisoned Angel: 12-20
10. Return to New Tristram: 12-20

Act 2

1. Shadows in the Desert: 12-20
2. The Road to Alcarnus: 13-21
3. City of Blood: 14-22
4. A Royal Audience: 15-23
5. Unexpected Allies: 15-24
6. Betrayer of the Horadrim: 17-25
7. Blood and Sand: 18-26
8. The Black Soulstone: 18-26

Act 3

1. The Siege of Bastion's Keep: 21-29
2. Turning the Tide: 22-30
3. The Breached Keep: 23-31
4. Tremors in the Stone: 23-31
5. Machines of War: 23-31
6. Siegebreaker: 24-32
7. Heart of Sin: 24-32

Act 4

1. Fall of the High Heavens: 25-33
2. The Light of Hope: 26-34
3. Beneath the Spire: 27-35
4. Prime Evil: 28-36

Nightmare difficulty

Act 1

1. The Fallen Star: 29-35
2. The Legacy of Cain: 31-36
3. A Shattered Crown: 31-37
4. Reign of the Black King: 33-38
5. Sword of the Stranger: 33-39
6. The Broken Blade: 34-39
7. The Doom in Wortham: 35-41
8. Trailing the Coven: 35-41
9. The Imprisoned Angel: 38-43
10. Return to New Tristram: 38-43

Act 2

1. Shadows in the Desert: 38-43
2. The Road to Alcarnus: 39-44
3. City of Blood: 39-45
4. A Royal Audience: 40-45
5. Unexpected Allies: 41-46
6. Betrayer of the Horadrim: 41-47
7. Blood and Sand: 42-47
8. The Black Soulstone: 42-47

Act 3

1. The Siege of Bastion's Keep: 44-49
2. Turning the Tide: 45-50
3. The Breached Keep: 45-51
4. Tremors in the Stone: 45-51
5. Machines of War: 45-51
6. Siegebreaker: 46-51
7. Heart of Sin: 46-51

Act 4

1. Fall of the High Heavens: 47-52
2. The Light of Hope: 47-53
3. Beneath the Spire: 48-53
4. Prime Evil: 49-54

Hell difficulty

Act 1

1. The Fallen Star: 49-52
2. The Legacy of Cain: 50-53
3. A Shattered Crown: 51-53
4. Reign of the Black King: 51-54
5. Sword of the Stranger: 52-54
6. The Broken Blade: 52-55
7. The Doom in Wortham: 53-55
8. Trailing the Coven: 53-55
9. The Imprisoned Angel: 54-57
10. Return to New Tristram: 54-57

Act 2

1. Shadows in the Desert: 54-57
2. The Road to Alcarnus: 54-57
3. City of Blood: 55-57
4. A Royal Audience: 55-58
5. Unexpected Allies: 55-58
6. Betrayer of the Horadrim: 56-58
7. Blood and Sand: 56-59
8. The Black Soulstone: 56-59

Act 3

1. The Siege of Bastion's Keep: 57-60
2. Turning the Tide: 57-60
3. The Breached Keep: 58-60
4. Tremors in the Stone: 58-60
5. Machines of War: 58-60
6. Siegebreaker: 58-60
7. Heart of Sin: 58-60

Act 4

1. Fall of the High Heavens: 58-60
2. The Light of Hope: 59-60
3. Beneath the Spire: 59-60
4. Prime Evil: 59-60

Inferno difficulty

Inferno difficulty isn't available until you reach level 60.