In order to save costs, you may use only App Insights for logging. However, please keep in mind that one does not replace the other fully, although there is some overlap.
- App Insights is a part of Azure Monitor, however, it acts as an automatic problem monitor as opposed to having a deep insight over your Azure infrastructure.
- If you do not require in your day-to-day activities a deeper insight of the APIs, such as telemetry analysis for example, then perhaps Azure Monitor is not needed.
There is also the option of setting for each specific API and/or APIM instance, so if you run into performance issues on one you might want to use Azure Monitor, as opposed to only using App Insights.
Without deeper insight on the specifics of your business and infrastructure, my advice is to continue with both for the production instance, and cut back on Azure Monitor for dev and staging if you'd like.