It depends on who wants it and why. A serious Labrador enthusiast would want the book because of the words inside of it. They don't care about dust jackets and the condition of the book. Besides comdition, demand and scarcity will also drive up the cost of a book. Search abebooks.com and alibris.com to get an idea of prices.
"Outside of a dog, a book is man's best friend. Inside of a dog it's too dark to read."
-Groucho Marx
You might want to also try amazon.com. I got my copy of The Dual Purpose Labrador on there for under $50. It's been quite a while, but worth a shot if that's what you're looking for.
The Dual-Purpose Labrador
Roslin-Williams, Mary
Bookseller: Heirloom Bookstore
(York, PA, U.S.A.) Price: US$ 172.50
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US$ 5.00
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Book Description: Pelham Books, London, 1979. HARDCOVER. Book Condition: VG. Dust Jacket Condition: VERY GOOD. Photographs (illustrator). 8vo. 5th impression. Lightly bumped at the tips, else a Very Good copy in a Very Good dust jacket. Scarce title! 5TH PRINTING ed. 192pp. Bookseller Inventory # AMA12580420
Book Description: H.F. & G. Witherby Ltd., 1988. Hardcover. Book Condition: Near Fine in Very Good DJ. DJ has slight shelf wear and scuffing to edges. Otherwise, pages are crisp and clean with no writing or highlighting! Very clean!. Bookseller Inventory # AJ09322rw
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2. ADVANCED LABRADOR BREEDING.
Williams, Mary Roslin.
Bookseller: Between The Lines
(Bloomington, IN, U.S.A.) Price: US$ 200.00
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US$ 4.00
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Book Description: London. UK H. F. & G. Witherby 1988., 1988. 2 board corners and spine heel very slightly bumped. pages unaffected, o/w vg+ in top and bottom edge wrinkled, but NOT torn or scuffed, o/w vg dj, 151 pp. sewn binding. Illustrated by 23 pp b/w/color photos. 1st ed. Binding is hb. Bookseller Inventory # 016289
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3. Advanced Labrador Breeding (ISBN: 0854931589)
Mary Roslin Williams
Bookseller: Terra Australis Books
(Daisy Hill, QLD, Australia) Price: US$ 243.15
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US$ 8.17
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Book Description: H.F. & G. Witherby Ltd, London, 1988. Hard Cover. Book Condition: Very Good. Dust Jacket Condition: Good. 8vo - over 7¾" - 9¾" tall. Usual library marks.151 pages with 11 colour and 27 b/w photos. Ex-Library. Bookseller Inventory # 003340
Of what I've noticed, Dual-Purpose is in less demand than Advanced Labrador Breeding. I got mine for $38 at Borders (off their website--they even paid for shipping, I just had to pick it up!) It used to belong to Kendall Herr, apparantly, as her name is in the front cover. It's pretty ragged, though. You can regularly get it for under $100 in readable condition. That's not counting hard-core book collectors who want mint with jackets.
Advanced Labrador Breeding is really now just for collectors since the new one came out, and the price IMO has levelled off at between $200 and $400, more for excellent or with dust jackets. IMO the prices have settled higher than you'd expect because with a collector, once they buy it, they aren't going to sell it. With a breeder, you buy it--then sell it when you get out of Labs. So more recycling, and better prices. I probably would never grab the original unless I found it at a STEAL and would still probably flip it.
I was under the impression that some of the photos and the measurement table in the back of the original were not in the Reaching for the Stars Edition?