The cost of a subscription renewal is typically the full price rate of the subscription, as advertised when you first signed up. If you were able to make use of a discount code when you started your subscription, the next billing period will be at full price (unless otherwise stated).
You can view details of your subscription, including renewal date and cost, in your subscription management settings.
My friend and I have the same subscription but at different rates. Why?
At Teach Starter we honour grandfather pricing, meaning that you will only ever be charged the rate your subscription was when you first signed up. Over time our pricing has changed to reflect improvements in the product, however, we ensure that you are only ever charged the maximum rate you agreed to when signing up (the total of the subscription, excluding any discount codes applied at initial purchase).
Your friend may have signed up in 2017 and yourself in 2019, and prices will have changed for new subscribers over this period to reflect the additional product offerings added over time.