Based on comparables of sales of other Victorian prints by Siegmund Hildesheimer & Co, the value of the die cut and treated prints would each be in the range of $30 - $50 per print. In a set of 12, the total value is $360 - $600, given that this may be 2nd known complete set, the value should double to $700 - $1200.
I do encourage and highly recommend you contact some various auction houses to also find out their opinions of estimate. It is free to contact them and you are not obligated to sell through any of them. In this case, the best are would be rare and antique book auctions, as this is in their wheelhouse.
Bauman's Rare Books in Manhattan - http://www.baumanrarebooks.com/
PBA Rare Book Auctions in San Francisco: http://www.pbagalleries.com/
Swann’s Auction Galleries: http://www.swanngalleries.com/bo_oks.cgi
Bloomsbury London: http://www.bloomsburyauctions.com/
Heritage Auctions: www.ha.com
Australian Book Auctions: http://www.australianbookauctions.com/
Dominic Winter Auctions: www.dominic-winter.co.uk/
Bonham’s Book and Manuscripts Dept: www.bonhams.com/departments/BOK/
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