I mod fairly slowly and carefully so I would say per pair of pads, it takes me 40 mins to do Part 1, 10 mins to do Part 2, probably upwards to an hour to do Part 3 (I'm terrible at sewing), and 5 mins to do Part 4. So in total about 2 hours with no breaks. If they are done in bulk, that could be cut down to 1h30m maybe, not sure though.
In terms of material costs, it's close to nothing (tape you can get in dollar stores, and scissors you would have already) more than the pads themselves.
So in that sense then if this is to be made into a legitimately profitable venture, payment for around 2 hours of fairly taxing work would be fair - something around 50-70$ (on top of the customer paying for round shipping costs, and for the pleather pads + velour pads, ofc) depending on the modder's expertise, efficiency, and greed. Lol.
Which makes me wonder how much the Chinese persons making these headphones earn by the hour . . .
No offense but I wouldn't pay anyone 25-35 dollars an hour for custom pads. I would, however, pay an extra 25 bucks for a better headband and softer pads for the HE-400s by default.
Given that I am fairly certain I don't want my leather pads, I will probably eventually 'mod' them just for the sake of discovering whether there is a worthwhile improvement in sound or not.