Hello,
HikaSubscription does use the final price of the product to perform the recurring.
If you do set-up a discount on the product price, that price will be stored in the order and that specific price will be used by the recurring plugin.
When there are multiple products in the order and some do not have the recurring, HikaSubscription will process the price for the recurring part.
But, in the other hand, when there is just recurring products in the order, HikaSubscription can simply take the final price of the order as the recurring price.
Thus, in your specific case, if you just have one product and a coupon, the HikaSubscription behavior would be to use the order total price as recurring price.
It would require some modification in the core (and a new setting) to force the processing of the orders even if there is just one product (or to do it when there is a coupon).
It would also require a new setting for the coupons to be able to know if they should be handle by the recurring. Because maybe you do have a coupon which would be apply for all your subscription...
The full implementation would require some time and that's not something we can provide as small patch in the forum.
But it might be possible to modify the core to deactivate the optimization and force the processing of the order's products to generate the recurring sum. It would require some investigation and multiple tests in our side but it would be the fasted way.
Regards,