User wants to know how to handle a per-diem or per day fee in Core? They charge clients a fixed fee + expenses for field visits. The fee varies based on the individual project because some might be out-of-state.
The best way to handle per-diem charges is to create a fee schedule and associate the agreed upon fee with the expense item called, say, Field Visits or Site Visits. The expense item should be marked as billable so that the expense entries are billed to the client. On the other hand, the Client Hours (time entries) associated with this can be set to zero to avoid duplicate charges.
You might also want to check out Billing day rates.