You'll have a tough time finding a "book value" on a sewing machine. Most of these vintage girls are really a "dime a dozen"...very few of them are rare. (Comparatively speaking)
If I was to sell my 1924 99, I'd most likely start with an asking price of $100, and be willing to take $50....she's pristine, in a bendwood case, and has the knee control and attachments.