The HeapTupleHeader (per row) is 23 bytes long, actual column data always starts at a multiple of MAXALIGN (typically 8 bytes). That leaves 1 byte of padding that can be utilized by the NULL bitmap. In effect NULL storage is absolutely free for tables up to 8 columns.
Basically, NULL values occupy 1 bit in the NULL bitmap. But it's not that simple.
The null bitmap (per row) is only allocated if at least one column in that row holds a NULL value. This can lead to a seemingly paradoxic effect in tables with 9 or more columns: assigning the first NULL value to a column can take up more space on disk than writing a value to it. Conversely, removing the last NULL value from the row also removes the NULL bitmap.
Physically, the initial null bitmap occupies 1 byte between the HeapTupleHeader (23 bytes) and actual column data or the row OID (if you should still be using that) - which always start at a multiple of MAXALIGN (typically 8 bytes). This leaves 1 byte of padding that is utilized by the initial null bitmap.