Minecraft wiki entry for Anvil_mechanics, associated text: Soul Speed III (12), Thorns III (12), Feather Falling IV (4), Depth Strider III (6), Protection IV (4), Unbreaking III (3), Mending (2)
Combining the items pairwise (with the boots in the first slot) the cost is 12+4+4+2=22.
The resulting items are: Boots (Soul Speed III), Thorns III+Feather Falling IV (16), Depth Strider III+Protection IV (10), Unbreaking III+Mending (5).
The cost of the second round of combination is 16+5=21, plus 4 for the penalties, totalling 25.
The resulting items are: Boots (Soul Speed III+Thorns III+Feather Falling IV), Depth Strider III+Protection IV+Unbreaking III+Mending (15).
The cost of the last step is 15 plus the two penalties of 3 each, totalling 21.
The overall cost is 22+25+21=68 levels.