The fastest way to waste money is to pour traffic into a page that does not convert. Before you spend a cent on growth, confirm with your numbers that visitors subscribe at a healthy rate and that subscribers stay. If conversion or retention is weak, fix that first — paid growth multiplies whatever is already happening, including the leaks.
Know your numbers well enough to say what a subscriber is worth to you over time. Without that figure you cannot tell whether any paid channel is profitable, and you are gambling rather than investing.