Formulas of Electrum and Score gain.

[Higurashi]

This information is taken from an old Wiki article written by Xenocidius and from looking at the game code directly. The article was lost as domains changed, but the information is preserved. I have not looked at the game code recently to confirm that the formulas are the same, but I have no reason to think they're different either. I've added some formatting and clarified a lot of the text to make it more readable as well.

AI0 (Trainer):
Enter fee: 0
If Won: 1 (score/coins)
If EM: 1 score + 2 coins
If Lost: 0 (score/coins) Counts as a lost in your W/L
Slot: 0

AI1 (Random):
Enter fee: 0
If Won: <= 4 (score/coins)
Formula: 1 score/coin per range of 1-24/25-49/50-74,75-99 hp
100 HP EM: 10 (score/coins)
Formula (coins): Set reward 5 × 2 = 10 coins
Formula (score): Set reward 5 × 2 = 10 score
If Lost: 2 (coins)
Slot: 1
Slot bonus: +5 coins if the first 2 or all 3 images match up per slot (Max 5 coins)

AI2 (Normal):
Enter fee: 5
If Won: <=9
Formula: 1 score/coin per 10hp. EM doubles it.
100 HP EM: 20 (score/coins)
Formula (coins): Set reward 10 × 2 = 20 coins
Formula (score): set reward 10 × 2 = 20 score
If Lost: 5 (score/coins)
Slots: 2
Slot bonus: +5 coins if the first 2 or all 3 images match up per slot (Max 10 coins)

AI3 (Elder):
Enter fee: 10
If Won: <=19 (score/coins)
Formula: Normal win: the first digit in your score will be 1 and the second digit will be the first digit in your HP. If <10 hp you will get 10 coins/score.
100 HP EM: 35 score/ 40 coins
Formula (Coins): Set reward 20 × 2 = 40 coins
Formula (score): Set reward + ¾ of reward (20 + 15) = 35 score
If Lost: 10 (score/coins)
Slots: 3
Slot bonus: +5 coins if the first 2 or all 3 images match up per slot (Max 15 coins)
 
AI5 (Half-Bloods):
Enter fee: 20
If Won: 20=><=39
Formula: Normal win: 15 + Hp/4 coins. EM: double set reward for coins and set reward + Hp/4 for score.
100 HP EM: 45 score/ 80 coins
Formula (coins): Set reward 20 × 4 = 80 coins
Formula (score): Set reward 20 + 25 (100/4) (HP / 4) = 45 score
If Lost: 20 (score/coins)
Slots: 3
Slot bonus: +5 coins if the first 2 or all 3 images match up per slot (Max 15 coins)

AI6 (False Gods):
Enter fee: 30
If Won: <=59
Formula: Normal win: 35 + Hp/4 coins and 65 + Hp/4 score. EM: double reward + 30 extra score.
100 HP EM: 120 (score/coins)
Formula (coins): Set reward 35 + Hp/4 x2 = 120 coins
Formula (score): Set reward 65 +Hp/4 +30 = 120 score
If Lost: 30 (score/coins)
Slots: 3
Slot bonus: +5 coins if the first 2 or all 3 images match up per slot (Max 15 coins)

Arena:
‘bonus’ is the base reward of the league that you win against.

However, if the deck is rank 4-9, bonus is doubled.
If the deck is rank 3, bonus is multiplied by 6.
If the deck is rank 2, bonus is multiplied by 8.
If the deck is rank 1, bonus is multiplied by 10.

Electrum: -5 + round-down(bonus / 2 + bonus * (HP / 200))
Score: -5 + round-down(bonus / 2 + bonus * (HP / 200))
If Lost: 20 (score/coins)
Regular Slots: 3 (Bronze: Unupgraded, Silver: Unupgraded, Gold: Unupgraded, and Platinum: Unupgraded.
Upgraded Card From Bonus Spin: Gold + Platinum.
Slot bonus: +5 coins if the first 2 or all 3 images match up per slot (Max 15 coins)
Special slots: 1 (Once all rings are lit)
Slot bonus slot changes: 3 Per slot. (Rare card)

EM: Double electrum gained. Gain 20 extra score.

PvP 1:
uberfactor = (your score – opponent’s score) / (absolute(your score) + absolute(opponent’s score))

Electrum: 60 + round-down(35 * (1 – uberfactor))
Score: 60 + round-down(35 * (1 – uberfactor))

EM:
Gain 20 extra electrum.
Gain 20 extra score.
Slots: 3
Slot bonus: +5 coins if the first 2 or all 3 images match up per slot (Max 15 coins)

PvP 2:
uberfactor = (your score – opponent’s score) / (absolute(your score) + absolute(opponent’s score))

Electrum: 100 + round-down(35 * (1 – uberfactor))
Score: 100 + round-down(35 * (1 – uberfactor))

EM:
Gain 30 extra electrum.
Gain 30 extra score.
Slots: 3
Slot bonus: +5 coins if the first 2 or all 3 images match up per slot (Max 15 coins)

[SpikeSpiegel]

+ rep

 

[Monox D. I-Fly]

Arena:
‘bonus’ is the base reward of the league that you win against.

However, if the deck is rank 4-9, bonus is doubled.
If the deck is rank 3, bonus is multiplied by 6.
If the deck is rank 2, bonus is multiplied by 8.
If the deck is rank 1, bonus is multiplied by 10.

Electrum: -5 + round-down(bonus / 2 + bonus * (HP / 200))
Score: -5 + round-down(bonus / 2 + bonus * (HP / 200))

So this is why sometimes I get a much larger sum of  in the Arena than usual? All along this time, I thought it was just some lucky bugs for me.

[Higurashi]

If it's a much larger sum, then yes; it's a top 10 deck. If it's a smaller difference, it's likely just the specific league adjusting its reward automatically according to how many people are playing against it. That said, this has also been known to bug out, but it always readjusts in time.

[Zawadx]

Any idea how base rewards are calculated? They might need some recalculation soon.

[CactusKing]

Any idea how base rewards are calculated? They might need some recalculation soon.

+1

Spoiler for Hidden:

[Monox D. I-Fly]

Why does almost every won duel in Gold Arena give zero Electrum now?

[Higurashi]

It happens periodically. As the formula is supposed to adjust after how many people play it, it can just keep going down and down (or up and up) until it hits a point it shouldn't be at. In the past, the rewards have always reset after a while when this happens. If it doesn't, I send a mail to zanz.

[Monox D. I-Fly]

Wow, I just got 1415 Electrum from a single duel in Bronze. 1 HP opponent, won only using Spark on my first turn. Got 1400 Electrum + 15 Electrum more from the double spins.

[TribalTrouble]

Wow, I just got 1415 Electrum from a single duel in Bronze. 1 HP opponent, won only using Spark on my first turn. Got 1400 Electrum + 15 Electrum more from the double spins.

It was likely one of the top 10 Bronze decks.

[Monox D. I-Fly]

Wow, I just got 1415 Electrum from a single duel in Bronze. 1 HP opponent, won only using Spark on my first turn. Got 1400 Electrum + 15 Electrum more from the double spins.

It was likely one of the top 10 Bronze decks.

So the top 10 has became 1 HP decks?

[CactusKing]

Wow, I just got 1415 Electrum from a single duel in Bronze. 1 HP opponent, won only using Spark on my first turn. Got 1400 Electrum + 15 Electrum more from the double spins.

It was likely one of the top 10 Bronze decks.

So the top 10 has became 1 HP decks?

Some of them will have. They're constantly changing