Yes, that is a very difficult question to wrap with a reply.
Most items with Lizzie's signature are letters, which are held by family and close close friends of the Bordens. I have seen a couple of books with actual full signatures of Lizzie and Emma, but those are rare.
Recently (in the last 3 years) a couple of Lizzie's books came up for sale on ebay, but those were initialed L A B. They sold between 300 and 800 dollars. (est.) A full signature should be much more. But again, that is hard to determine since there are so few that have come up on the market.
Ebay does not really reflect the true worth of an item, and good deals can be had there. Most items of value on ebay sell for about 40 to 60 percent below their real worth. Place Lizzie's signature in a New York or London Auction house and it could be very high.
In today's economy it's hard to say.
A letter, note, or book owned by Lizzie, with her signature in it, would be worth a lot more than just her signature written on a detached piece of paper.
As a collector, a contemporary tome, novel or non fiction, in very good condition and with Lizzie Borden's full signature in it would be worth upwards of 12 hundred dollars........at least to me.
