Please refer to this page for Tesla’s official description of AutoPilot features vs. FSD.
It is important to note that ‘Full Self Driving” is not totally autonomous driving, nor will it be for several years to come (barring any Elon Musk-style major breakthroughs). As the driver, you are always responsible for your vehicle and must pay attention at all times.
If you do not go on long trips or do not wish to have the additional features listed on the Tesla page above, then perhaps spending money on FSD may not be worth it for you. This is mostly an investment in the future as well, but that future could be years away.
As of July 2021, you can get a subscription service for FSD for $199 per month, through the Tesla application.