The 'woocommerce_subscriptions_product_price_string'
filter can be used to customise the price displayed for a product.
The 'woocommerce_subscription_price_string'
filter can be used to customise the cart/order prices displayed in the cart and on an order.
For examples of how to build your price strings and details of the parameters passed to each filter, view the source of the WC_Subscriptions_Product::get_price_string()
and WC_Subscriptions_Manager::get_subscription_price_string()
functions. I always try to write self-documenting code & include PHPDoc for anything that is not self evident.
Hope that helps. :)