Worth more/less than scrolls?
Like magic items, I think we have to operate with two prices: PC buying price and PC selling price. Generally the buying price is twice the selling price, with the difference representing the efforrt/time expended by the middle man (magicshop-keeper). Here, I will look at the PC selling price, the price a PC can get by selling to 'anyone'´:
ValueToBuy = ResalePrice + PersonalGainFromContents
PersonalGainFromContents is the value of using the spells for scribing and that of using the spells as a spellbook.
The value of scribing is difficult to evaluate and really depends on the rarity of the spells in question in the campaign. In effect, a table like the cost-to-cast table in the DMG is needed.
Any way, the ScribingValue should be no greater than the scroll-cost of the spell - a scroll can be also used to immediately cast the spell and the spellbook cannot.
HERE is the main proposition of this post: ScribingValue = X*ScrollValue
Assuming ValueToBuy=0.5*ResalePrice (or 2*ValueToBuy=ResalePrice) as above, the proposition leads to
ValueToBuy = 0.5*ValueToBuy + X*ScrollValue
or
ValueToBuy = (0.5/X)*ScrollValue (remember... adjusted for components)
With X=0.75, this means that ValueToBuy = 2/3*ScrollValue
With X=0.5, this means that ValueToBuy = ScrollValue
Of course, this should be calculated for each spell (easy). The main thing is to remember that the trade-value of a spell-book should be no greater than this.
Edit: Errors are likely... I'm tired
Edit2: What is really needed is a rule in the DMG. It only has to state something like "spellbook value = scroll value of individual spells at minimum castilg level"... That would be satisfactory, simple and quite reasonable.