My DM & I have instituted this maxim - the spell cannot be more expensive than buying it as a scroll at minimum level (of course, barring market inflation and exactly who you are buying it from. That covers the spell's base price.
Then scribing it into a "standard" non-magical 100 page book costs 50 g.p per page per spell level (cantrips using only a single page) [our campaign has new cantrips, and as such, not every wizard knows every cantrip...I think he gives us our choice of 2xInt mod at 1st level]. This formula makes it pretty close to 2e spellbook cost (1000 gp per book + 50 gp per page in ink).
Of course, I personally use Boccob's Blessed Books for my scribing, and since it doesn't exactly say how much it costs to scribe we make each page cost 50 gp. Pretty economical and the extra-sturdy/less encumberance BBB is a nice addition.
Hope this helps.
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