"What running costs do they incur if Lemmy is free and open?"

The time they spend to make a product better. That is extra money they can use to put more effort into the app.

Sync's edge is the features it has, the customizable UI and a great developer that listens to its willing consumers that will pay to continue making that app better.

"I saw a subscription model for basic features..."

See this is my issue. The app is usable. You have the choice to pay. So don't if you don't want to. The API and features you want aren't here for a cost and that is fine.

You know what though? I will keep paying for those features a smoother Lemmy experience. An experience that brings more people from Reddit that gives the end user a smoother and much more impactful first impression.

If you were to hand me Jerboa and Sync at the same time and let me look them over. I would still pay for Sync. If you were to tell me which app I would recommend to a new user. I would tell them Sync. Why? Because Sync just over all functions and runs better. The UI is better. The little featured to tweak your experience are better. Navigation is better. I can use Sync on a tablet or larger device.

Sync creates a better first impression. People use things that look nice, work even more nicely. Here is the real shocker, they will even pay for those things.