In my personal experience silver boxes have given me greater rewards than gold boxes, determined by how much coins you spend by opening them and by how much the box is worth.
I've opened I think 3 gold boxes so far; I have sold to npc the rest. The rewards I got were:
1. 23 Enhancement Stones
2. 24 Enhancement Stones
3. 17 coins
Now, when you look at treasure --> buy treasure, a gold box costs 20 coins and sells for 40k money to the npc. So 20 Coins = 1 Gold Box = 40,000 money. Also, 1 coin buys 2000 money. So 1 Coin = 2000 money.
1 Coin = 1 Enhancement Stone, 1 Enhancement Stone = 2000 money (npc) so... totalling it all up in terms of money:
1 Coin = 2000 money = 1 Enhancement stone.
When you open a gold box or a silver box, you have to look at the npc price and determine the box's value and the value of the coins used to open the box to the reward you got from opening the box. So, looking at what I got from the first one (23 Enhancement Stones) 1 Gold box = 40 thousand money = 20 coins. 10 Coins used to open the box = 30 coins usage in all = 60 thousand money OR 30 coins. So it cost me 60 thousand money approximately if you convert them all to money.
I received 23 Enhancement Stones from the box, and we know as stated above 1 Enhancement Stone = 1 Coin = 2000 money. So 23 enhancement stones = 46 thousand money. 1 Gold Box + 10 coins to open it = 60 thousand money. Altogether, I lost 14 coins or 28 thousand money on the deal.
The second one, 24 enhancement stones, is just 2000 money more, so I lost 26 thousand money on the deal instead.
The last one was 17 coins. Here we just convert them into coins instead of money to make it simpler. 1 Gold Box = 20 Coins + 10 Coins to open it = 30 Coins. I received 17 Coins. Altogether, I lost 13 coins on the deal.
Although some people would say that earning 7 coins (because you opened it for 10 and received 17) is a good deal since coins are very rare, I would disagree just because of the calculations stated above. Although I do know some people have different opinions.
I have opened 4 silver boxes in my lifetime also, and I received:
1. Scepter
2. Nugget
3. 18 thousand money (I think)
4. Some amount of enhancement stones, I forget how many though. xD
So. 1 silver box + 5 coins = 15 coins, 1 Scepter = 80 coins. So I basically earned 65 coins on the deal.
A nugget lost 10 coins on the deal.
18 Thousand money (I think) lost 12 thousand money on the deal.
Because of this, I have sold every box I have gotten to the npc because of the money for the techs.
But it could just be my bad luck.

Edit: I'm sorry about the long post guys.
