But in ACSC there are stamps listed that are very rare but have not that value. For example, the Victoria 1901 5d stamp (brown), ACSC V90-A-aa, has the variety watermark reversed, value 750$. The notes says that there is 1 (one) stamp known. Other examples in the ACSC show similar prices, e.g. NSW ACSC N14aa.
Catalogue value is very difficult for such rarities and may well have been based on past auction sales for the same or similar items.
Past sales means nothing, of course, if in the future there are two wealthy bidders keen to add a rarity to their collection - the sky's the limit.