There is no "Law" that says you MUST have a storefront, and nothing anywhere about a dollar figure to have a 01 FFL.
The BATF does want FFL holders to be actually "In the business", not hobby Dealers who have a license for their own personal benefit.
Years ago, we were told by the Feds "Get a FFL. It'll keep you legal, and you can save money".
Then Chucki Schumer threw a hissy-fit about "these hobbyist who just have a FFL to save money". (Apparently, saving money upsets liberals).
So then, under Billy Boy Clinton, the BATF started instituting all kinds of changes for qualification for a FFL, and made it much harder to have one.
The price went up, (Not as much as the Democrats wanted), they started requiring photos and fingerprints, and demanding photos of "your store".
IN any event, while not FORBIDDEN, it's tough to get an FFL unless you have some kind of storefront.
Much of this depends on where you live. In the mid-West, the BATF doesn't much care, in more liberal areas, even store front operations are getting a lot of picky scrutiny.
The BATF will also demand that you meet ALL state and local laws, ordinances, and tax and licensing requirements, like having a tax number and business licenses.