Merging them has a random chance of producing only one of these objects: Magic Coins - Gold, Dragon Egg Chest/Dragon Nest Vault (Midas Ducks variant) costing 17 and 85 gems respectively to open, and Golden Apples of the next level.
Price wise, it is recommended to sell the level 1 apples. If you wait and merge Level 3 apples you get 1 nest chest and 2 egg chests, equal total of 80 coins, while 3 level 3 apples are worth 90 coins. However, 3 Level 2 apples give 45 gold (much better than one L3 apple), and 3 L1 apples is 24 gold, again much better than a Level 2 apple.
The recommendation above is true if you don't have space in camp. If you do, merge always 5 of L1 and L2, and sell only L3 and the boxes you get along the way. The real cash would come from the Gold Coins you would accumulate and merge as well, as merging Gold Coins into higher tiers will create even more coins.
A quick study of merging golden apples into Giant Golden apple yielded 3 egg chests, 15 giant golden apples, and 7 golden coins. Merging those 15 golden apples yielded only Cosmos and egg chests (and one Midas Duck nest vault). And merging the Cosmos eggs yielded only chests. That gives golden coins at approx 30% proc rate. Or another way, each Golden Apple is 0.10 a golden coin. 25 bronze coins gives you 4 golden coins, or 0.16. So each Golden Apple is worth slightly less than a bronze coin, not counting selling the chests. A more detailed study gave these results:
From merging Golden Apples: 363 giant golden apples, 193 gold coins, 113 egg chests, 56 egg vaults.
From merging Giant Golden Apples: 74 Golden Apples of the Cosmos, 17 gold coins, 35 egg chests, 21 egg vaults
In both cases, about 50% of the products are the larger apples. But it looks like 1/4 of the merge products of the small apples are gold coins, while only 1/8 of the products of the large apples are. And the rest generate chests and vaults in a 2:1 ratio.
Every merge of Golden Apples of the Cosmos appears to generate two egg chests and an egg vault.
|1||1x1||Epic||Golden Apple||Fruit from King Midas's Garden.||8|
|2||1x1||Epic||Giant Golden Apple||Touched by a Midas Duck.||15|
|3||2x2||Legendary||Golden Apple of the Cosmos||This fruit has a strange, goldeny-magic about it.||30|