Black women's hair appointments are more expensive; many hair salons charge a premium. Plus they use a lot of products and elixirs to change the nature of their hair. Plus, many of them wear a wig-like accessory, often of real human hair, every single day. The process of 'weaving' that into their natural hair can cost hundreds or even thousands, at the high end.
Source: black friend in college wrote an essay about this for a journal I was editing at the time, plus, common knowledge. There are some interesting cultural questions about how in North American society, natural black hair is deemed unacceptable, and this has generally been internalized by black women. If you choose to just be natural (as my friend did) you get a lot of flak from other black women.
I don't know if you can get this to work out to $500B; other industry numbers I could find are like $10B, which would be about $50/month for every black woman in the USA.