There's a fixed cost to put a transaction onto the card brand networks. Recurring includes dunning management which retries failed auths. If you rolled your own, you'd be paying for the declines.
Stripe is charging extra 0.5% for the billing feature, so a billing charge goes in the US with stripe 3.4% + 30¢. It could be done for 2.9% + 30¢ + 5¢, stripe will still get the its cut, but not the extra 0.5%.
Sounds like you could clean up in this space at $00.001 per transaction!
¢5/transactions for billing services we could do it, might need some small seed funding to set up a small dev team. Do you think we could get interest customers?
You could get customers, but you'll quickly learn they ask for very complex features, and you'll struggle to pay your dev team with 5 cents per transaction.
Complex customers can use 0.5% extra service from Stripe, they could even build their own solution. Billing, recurring, invoicing can be done here. What could be interesting is setting up a billing service that the client brings its own Payment Service (checkout.com, stripe.com, adyen.com).