How much does a Shopify mobile app cost?

It depends on the route. A no-code app builder is a monthly subscription that never ends. A done-for-you build is a one-off project fee. A fully custom app from an agency is the most expensive by a wide margin.

The costs nobody mentions are the store fees: an Apple Developer account is an annual fee and a Google Play account is a one-off, and both are yours rather than the builder's.

The other one is time. A cheap tool that needs a day a week of your attention is not cheap.