Monthly fees fund the processor's account maintenance, statement generation, and basic support. They range from $0 (Stripe, Square, Helcim) to $30+ (typical for traditional ISOs and bank programs). Subscription pricing models like Stax flip this on its head: you pay $99–$199/month and get interchange-plus with no percentage markup — which makes sense above $25K/month volume but is overkill below.
Watch out for hidden monthly fees that don't appear in the advertised rate: PCI compliance fees ($10–$25/mo), gateway fees ($25/mo via Authorize.net), statement fees ($5–$10/mo), batch fees (~$0.25 per batch close). These can add up to $50/mo of pure overhead before you process a single dollar.