Go to the Phone icon which you will see along the menu bar.
This takes you to the Call Records page as shown below:
Select from the 'All Call Types' dropdown - 'SMS messages only'
You will now see a list of SMS messages sent in the month you have selected with details of the cost per SMS as shown below:
You can also use the search field to enter in a mobile number to search for all SMS messages that have been sent to that number.
Please note: if you do not see the results for this number try removing the '0' from the start of the number to then see the results of how many SMS messages have been sent.