For me the easiest way is:
Copy the whole range you are planning to drag onto including the draggable item (select+ Ctrl+C)
Drag the item and break the formatting
Select the range and paste format only (Paste Special - Paste format only Or Ctrl+Alt+V)
Custom number formatting supports conditionals, but only two of them plus a default format. So, to display millions and billions of dollars, you can use Format > Number > More Formats > Custom number format with something like this:
[>999999999.999]$#.00,,, "B";[>999999.999]$#.00,, "M";$#,##0_M
Unfortunately, this will ...