It's all about rarity.
A book signed by the authors in the name of some other luminary can be very valuable indeed. But signed in the name of an unknown can reduce the value.
Best just to get the book signed without any form of dedication.
I have over 150 first editions so signed. I wonder what they are worth?
One thing I can tell you is that my own first edition of this book is now worth about double its original value. The first edition has sold out, and that makes it rare.
The most rare angling book I ever possessed was a signed review copy of Harry Plunket-Green's "Where The Bright Waters Meet", which was sent out by the publishers to the then chairman of The Cape Piscatorial Society,ca 1922. This book was pristine and was given to me in 1978 by a committee member of the Rand Piscatorial Association. Like a fool I gave it away ca 1981.
I have since learned that this book recently would probably fetch many thousands of pounds at auction. It was unique - a one-off, and one of the finest and most evocative books on angling ever written.
Oh and buy the way, a recent signed first edition of Ian Fleming's "Casino Royale", went for £29,000 at Sotheby's I believe.