The wiki calculation was confusing to read, so I tested it myself when I was feeding +eggs to my ADK a while ago.
In comparison to the cost for the +stats, the cost for your monster level is really negligible.
Here are a few examples, using J0dy's Horus at Lvl.94 and +59 stats.
Example 1: J0dy feeds his Horus a single +50 monster.
9400 gold for Horus' level x 1 fed monster = 9400 gold
59,000 gold for the existing +59 stats on Horus = 59,000 gold
50,000 gold for the +50 stats being fed to Horus = 50,000 gold
Total = 118,400 gold
Example 2: J0dy feeds his Horus five +10 monsters at once.
9400 gold for Horus' level x 5 fed monsters = 47,000 gold
59,000 gold for the existing +59 stats on Horus = 59,000 gold
50,000 gold for the +50 stats being fed to Horus = 50,000 gold
Total = 156,000 gold
Example 3: J0dy feeds his Horus five +10 monsters, one at a time.
(1st feeding) 9400 gold for Horus' level x 1 fed monster = 9400 gold
59,000 gold for the existing +59 stats on Horus = 59,000 gold
10,000 gold for the +10 stats being fed to Horus = 10,000 gold
Sub-total = 78,400 gold
(2nd feeding) 9400 gold for Horus' level x 1 fed monster = 9400 gold
69,000 gold for the existing +69 stats on Horus = 69,000 gold
10,000 gold for the +10 stats being fed to Horus = 10,000 gold
Sub-total = 88,400 gold
(3rd feeding) 9400 gold for Horus' level x 1 fed monster = 9400 gold
79,000 gold for the existing +79 stats on Horus = 79,000 gold
10,000 gold for the +10 stats being fed to Horus = 10,000 gold
Sub-total = 98,400 gold
(4th feeding) 9400 gold for Horus' level x 1 fed monster = 9400 gold
89,000 gold for the existing +89 stats on Horus = 89,000 gold
10,000 gold for the +10 stats being fed to Horus = 10,000 gold
Sub-total = 108,400 gold
(5th feeding) 9400 gold for Horus' level x 1 fed monster = 9400 gold
99,000 gold for the existing +99 stats on Horus = 99,000 gold
10,000 gold for the +10 stats being fed to Horus = 10,000 gold
Sub-total = 118,400 gold
Total = 492,000 gold
For simplicity's sake, I just assumed Horus didn't gain any levels between feedings. It wouldn't make much difference in costs anyway. Obviously, Example 3 costs substantially more gold than the first two examples, because for each separate feeding, J0dy has to pay 1000g for every existing +stat on Horus. It's like compound interest on steroids.
A balanced compromise that I use myself is I feed five +1 monsters to a sixth +1 monster to create a single +6 monster. When I accumulate five +6 monsters, I then feed them to whatever monster I'm building up. So, I feed the main monster +30 stats at a time.