Q: Under home sharing, is there a way to have any single app for one of the accounts billed separately?

Hi Midgetkb,

I understand that you have questions regarding payments handled during Family Sharing. Let's go over how Family Sharing billing works. 

The way that Family Sharing Woks is that the organizer is responsible for all the purchases made for the entire family. Now it is not to say that you would have to pay for everything. What can be done is you can apply an iTunes Gift Card to the account that would like the app. The way items are billed is that it will take the iTunes Credit first before being billed to the card that is on file. Take a look at the section below that explain this in more detail. 
 

Making purchases

After you set up your family, any time a family member initiates a new purchase it will be billed directly to your account, unless that family member has gift or store credit. First, their store credit will be used to pay the partial or total bill. The remainder will bill to the family organizer's card. As the family organizer, any receipts generated by the transaction will be sent to you. Learn more about how iTunes Store purchases are billed

Family purchases and payments
https://support.apple.com/en-us/HT201079

Have a good day.