I don't think it costs Apple much to do this.

How much does it cost to keep Logic around? There's engineering/product/design costs, marketing, and training/selling costs in store. The training probably pays for itself and not many staff at Apple Stores are trained anyway. There's very little marketing, so it's entirely possible there is no dedicated marketing resource for Logic.

As for the product/engineering/design, it's well known that Apple is pretty lean on these sorts of things and often has far smaller teams than outsiders expect. I could see this easily being no more than a team of 5 at this point, maybe less. It obviously took a lot more to get it to this point, but with very few features over the last ~5 years, it's possible that it's been iterated enough to just be easy to maintain in its current form (assuming no major changes).

All in maintenance team budget could be <$2m/year (not including store training in this). Apple probably spent that on door handles in the new HQ, and I can see Logic being as important a "halo" project as the HQ door handles.