If that rumor price of $199 per month for FSD subscription is true, it would be about $2400 per year or just over four years time to the $10K price up front. But then you will have to keep on paying, meaning if you keep the car ten years, you would pay $24K for FSD, $14K more.
IMO, full FSD is not reality yet. Musk surely knows this. He is trying though. The system seems to be frequently confused by real world roads and traffic at times, more so with multi lane access and ramps with missing lane lines. Hoping that the new Tesla Vision makes real FSD sooner and faster with AI in use... and our helping by driving as Beta testers in our new MY using TACC and Autopilot.
If you believe in and trust FSD (and want to spring for another ten grand), seems better off that you buy FSD now using todays dollars at a locked in price.