According to my Scott Specialized Catalog (2011) the value of UC9 (which is really a Scott U525 envelope with the 6c Revalued Overprint) is $65 (mint) and $35 (used) for the cut square; the entire envelope is valued at $90 (mint) and $45 (used). It is also noted that ten surcharge varieties are known to exist.
While the value of this cut square is probably more than a typical one, there are not that many collectors who want cut squares, preferring instead the entire cover. That alone may limit the interest in the item and will reflect on secondary market values. I would also say that whoever wrote the "29" on the cut square you scanned, has probably diminished the value of the item to some degree also.
I am not a specialist in US Postal Stationery, but since values are heavily based on supply and demand, that would have influence on the sale price. At present,
ebay has three or four listings for the UC9 "mint" cut square at a BIN price of $37.50 to $47.50, but that's with no bids, so as we all know, one can ask anything for such an item, the real value is what one will finally purchase it at, and I would suspect these will go for much less when an
ebay transaction is actually made.