How much does a Braintree Payments and Stripe transaction cost?

Just like ANY credit card processor that a vendor uses, there is a cost to the transaction. No one sees this transaction other than the payment processing company. If you buy soda at a gas station or a burger a restaurant the vendor pays out to the processing company. 

Braintree Payments charge 2.9% plus $0.30 per transaction. 

https://www.braintreepayments.com/

Stripe Payments charge 2.9% plus $0.30 per transaction. 

https://stripe.com/us/pricing