Hi Ardi,
Thanks for taking the time to submit this feature request.
The reason for the current design is so that the price the client is quoted on the order form during signup and on order confirmation emails does not vary from month-to-month.

It is an important legal requirement in some countries that the renewal price does not vary from that promised upon signing up.
Furthermore, subscription-based payment gateways such as PayPal and 2Checkout cannot vary the amount charged, so clients may over-underpay each month if this were the case.

These are a few of the hurdles to implementation which would need to be overcome. If there is the demand then it's potentially something we can consider.