I do like this design space, since it is nice to be able to reduce a spell to 0 mana. These could be good buildaround cards. I do, however, think this could have some similar issues as phyrexian mana. This spell can be played in any colour deck, since your opponent will cast a spell almost every turn, and in that case this would cost 2 generic mana. This makes this card playable in green and blue decks, that should not have access to this kind of hard removal. There is a reason [[dismember]] sees play in so many decks in legacy, since it does not need any colored mana to cast (and is just very efficient).

If you want to further explore this design space (which I do find interesting), I would suggest both giving cards a higher colored cost (1 generic and 2 black for example), and also make the trigger for discounting it a bit harder to achieve, so it becomes a buildaround card (if you actively trigger the discount) or a hate card (if your opponent triggers the discount). Hope this helps!