Liberty charges you Β£25 every month. Β£20 of this is added as credit to your account, which you can spend on anything at Liberty.
In terms of the Beauty Drop goodies, every 3 months, Liberty ship a box to you. So each box costs Β£15. The next box is in October.
IMHO If you want the calendar and the Beauty Drop, it makes sense to sign up and save the credit. In the past few years, Liberty also gave a Β£25 discount on the calendar to Beauty Drop members. If you only want the calendar, then sign up for Beauty Drop, buy the calendar with the discount, and cancel once the 3-month minimum period is up.
You might want to think of what you'd use the credit on if you don't like the calendar's contents. Beauty Drop members also get a 10% discount on edits, which helps if you like their edits.
There's a code floating around for Β£20 for the first 3 months, which makes your first box free.
Please ask if you have any more questions, happy to help!