From NRA-ILA:

Joe Biden – ludicrously marketed to Middle America as a “moderate” – plans to ban the manufacture, sale, and possession of a host of popular semiautomatic firearms, including the most popular one of all, the iconic AR-15. For those who already own such guns, Biden would extend an offer that could not be refused, except on pain of federal imprisonment: surrender them to the government for whatever compensation might be offered or pay $200 to register them in a national database. This $200 per unit tax would also apply to each firearm magazine that could hold more than 10 rounds.
So, for example, the owner of a single AR-15 with 5 standard capacity magazines who wanted to keep his or her property would be hit with a $1,200 tax bill. And this doesn’t even take into account any other registration and fingerprinting costs.