I don't find the sleep a big issue at all. Was just listing it as another minor benefit if the catch rate is 100% it will save time, wasted balls and reduce sleep risk to 0. Currently I do Apricorn runs and get a few fast balls for the base price. But yeah if you wanted to do it on mass I don't think the extra cost of the balls would be worth it. The other day I was seeing them go as low as 2k which might make them worth, but their won't be enough supply to stay at that price as you say.
Was just putting the idea out there in case you or someone else can think of a way to optimise it. 31 SPD for ATK IV dittos go for over 5x if they have adamant atm. But looking at the pokeball options to me it seems there isn't a way to utilise Espeon and keep the catch rate high for cheap. Also thanks for the guide, it's a great method.