Each keyword response costs the same as 1 text message being sent from your account. This price varies depending on your plan.
The inbound text you receive is not charged but the automatic response to the contact will be charged a credit. Any outbound message will cost a credit to be sent.
To see what each credit or keyword costs with your plan, go to My Account > Billing.
Please note that MMS messages may also cost more depending on your plan. Learn More.