Value, of course is market-determined, as stated by Wilfried.

Smaller book dealers I know do their pricing by checking Abebooks, trade journals, Amazon and other sites where offers are made.
No longer can out-of-print titles of collectible genres be purchased from a legitimate dealer for pocket change. If you want it enough, you pay the price.   
Occasionally I find a gem at church jumble sales, etc. My collection of old western song books is small because those by the old campaigners, like Powder River Jack Lee, seldom show up, and when they do, prices are often beyond what I want to pay.
Generic cowboy songbooks by Amsco, etc., are usually cheap.

Incidentally, one Bill Boyd title is offered through Abebooks by a dealer in Ohio, "Folio of Western Songs No. 1," $17.00. It won't be there long. Boyd, aka "The Cowboy Rambler," died in 1977. No copies of the very early "The Cowboy Sings" are on offer.
There are other songbooks by 'Bill Boyd(s)', but not the same person. Guest obviously knows nothing of the genre.