As a minimum baseline, the wizard who penned the spellbook needs to be recompensed for time (lifestyle) and materials, and wants to turn a pretty profit.
For each level of the spell, the process takes 2 hours and costs 50 gp. Assuming wizard wishes to maintain comfortable lifestyle, that's 2 gp per day (or 8 hours of work).
And then there are materials – we'll say the base spellbook (50 gp) and whatever the ink cost will be. One ounce of ink (10 gp) = 30 ml, you can get about 20 pages per ml, so 30 ml (1 oz) gives about 600 pages. There's no guideline in 5e for how many pages a spell takes up, but in past editions it was one page per spell level. So any single spellbook only needs 1 oz of ink.
We'd also assume higher level spells are more valuable because higher-level casters are rarer than low-level casters. A multiplier can used for the spell level. 1.0 for cantrips & first-level, 1.2 for second-level, 1.3 for third-level, and up to 1.9 for ninth-level.
Let's take a 9th level wizard who penned the following spellbook:
- 1st Level (x6): Alarm, Burning Hands, Charm Person, Chromatic Orb, Color Spray, Comprehend Languages
- 2nd Level (x5): Alter Self, Arcane Lock, Blindness/Deafness, Blur, Cloud of Daggers
- 3rd Level (x6): Bestow Curse, Blink, Clairvoyance, Counterspell, Dispel Magic, Water Breathing
- 4th Level (x6): Arcane Eye, Banishment, Confusion, Conjure Minor Elementals, Control Water, Ice Storm
- 5th Level (x4): Animate Objects, Bigby's Hand, Cone of Cold, Teleportation Circle
Cost maths would be:
60 gp materials costs
1.0 level multiplier * (1*6*50) level of spell * # spells * 50gp + (2 * (total hours / 8) comfortable lifestyle = 1.0 * 300 + (2 * (12 / 8)) =
303 gp
1.2 level multiplier * (2*5*50) level of spell * # spells * 50gp + (2* total hours / 8) comfortable lifestyle = 1.2 * 500 + (2 * (16/8)) =
604 gp
1.3 level multiplier * (3*6*50) level of spell * # spells * 50gp + (2* total hours / 8) comfortable lifestyle = 1.3 * 900 + (2 * (36/8)) =
1179 gp
1.4 level multiplier * (4*6*50) level of spell * # spells * 50gp + (2* total hours / 8) comfortable lifestyle = 1.4 * 1200 + (2 * (48/8)) =
1692 gp
1.5 level multiplier * (5*4*50) level of spell * # spells * 50gp + (2* total hours / 8) comfortable lifestyle = 1.5 * 1000 + (2 * (40/8)) =
1510 gp
For a total price of
5,348 gp.
Now comes the question of how much profit the wizard wants to turn. This is a niche product, highly valued by specific individuals, and there's a reason the wizard is doing this rather than baking bread. So a 20% profit margin doesn't seem unreasonable.
So the selling price might be around
6,417 gp and
6 sp.
If you go by the
wealth by level breakdown that was done by
@tankschmidt, then a 9th level PC would have amassed ~16,200 gp over the course of their career thus far. So purchasing this spellbook represents a major investment, on par with establishing a guildhall/trading post (5,000 gp), buying the majority share in a sailing ship (10,000 gp), or negotiating a very good price on a rare magic item (2d10 x 1,000 gp).