Stripe charges between $5 and $25 (maximum) per ACH transaction [1].

I'm curious, what costs do banks have to process a transaction, and what would determine their price?

This example is facetious, but Venmo is free and they deduct directly from my bank account too.

Is it just the costs of keeping their servers running, or it is some transactional expense, or liability, etc.?

[1] https://support.stripe.com/questions/pricing-of-payment-methods-in-the-us