Each plan includes a number of free SMS credits:
Pro - 50 free SMS
Grow - 25 free SMS
After this, there is a transactional charge of 7p per SMS charged via Stripe.
To view your current SMS spend:
Go to Menu >> Payments >> Email & SMS Invoices
This will display your SMS and Marketing Email spend by each week
Click view details on your desired week to view the full breakdown
Tip - Remember, you can choose which specific SMS updates you choose to send and these can be toggled on/off at any point.