How much does an ACH cost?

Could you please elaborate on the cost associated with an Automated Clearing House (ACH) transaction? As a financial practitioner, I'm curious about the general expenses involved in this type of electronic payment system. Specifically, I'd like to know if the cost differs for inbound versus outbound transfers, or if there are any additional fees for expedited processing. Also, does the cost vary depending on the financial institution or the type of account being accessed? Understanding these details would help me provide more accurate financial advice to my clients.