Okay. A 1st level Wizard gets all core 0-level spells in his spell book for free: that takes up 20 pages (in a 100-page book). Plus, he can select 3 1st-level spells plus 1 additional spell per point of Intelligence modifier (which is a maximum of 20 for an elf, human, half-orc, or half-elf Wizard). Let's say he is a 20 Intelligence Elf Wizard, so he has 8 1st-level spell, for 28 pages.
At each additional level of experience, the Wizard gets TWO (2) free spells of any level that he can cast to add to his book.
That means (assuming he always picks his highest level spells), he will have used 30 pages at 2nd level, 34 pages at 3rd level, 38 pages at 4th level, 44 pages at 5th level, 50 pages at 6th level, 58 pages at 7th level, 66 pages at 8th level, 76 pages at 9th level, 86 pages at 10th level, and 98 pages at 11th level.
Now he has to purchase a second spell-book. 12 pages at 12th level, 26 pages at 13th level, 40 pages at 14th level, 56 pages at 15th level, 72 pages at 16th level, and 90 pages at 17th level. He could add one of free spells at 18th level for 99 pages. And then it is on to Book Three.
9 pages at 18th level, 27 pages at 19th level, and finally 45 pages at 20th level.
He has spent 30 gp and is carrying three 3 lb spell books. Unfair! (you say).
He knows four (4) spells of every level 2nd-9th, plus 10 1st-level and 20 0-level. Now, if he wants to add new spells to his spell book, he has to pay to scribe these spells.
It costs 5 gp for a single 0-level spell, 10 gp for a single 1st-level spell, 40 gp for a single 2nd-level spell, 90 gp for a single 3rd-level spell, 160 gp for a single 4th-level spell, 250 gp for a single 5th-level spell, 360 gp for a single 6th-level spell, 490 gp for a single 7th-level spell, 640 gp for a single 8th-level spell, and 810 gp for a single 9th-level spell.
Not exactly chump change, eh?
But wait! How is the Wizard finding those spells? As DM, you control the treasure; if you don't want him to get spells on the cheap, then don't put arcane scrolls in the treasure. Now he has to buy those scrolls, and that get's expensive really fast.
Scrolls of 0-level cost 12 and a half gp, 1st-level scrolls cost 25 gp, 2nd-level scrolls cost 150 gp, 3rd-level scrolls cost 375 gp, 4th-level scrolls cost 700 gp, 5th-level scrolls cost 1,125 gp, 6th-level scrolls cost 1,650 gp, 7th-level scrolls cost 2,275 gp, 8th-level scrolls cost 3,000 gp, and 9th-level scrolls cost 3,825 gp. For one spell.
Or he can conduct independent research, taking a week for a single spell and costing 1,000 gp per spell level! GAH!
But wait! He can copy spells from another caster's book. Yes, yes, he can. If you as DM let him find a wizard willing to share his knowledge of the secrets of power. Most aren't, unless they are getting something very tangible and very useful in return. He could kill spellcasters and try to use their books, but Wizards have long been known in this genre to trap and ward their spellbooks--and whose to say they didn't deliberately put a spell in there designed to backfire on a thief? Hmmmmm? Why that would be you as the DM, wouldn't it?
Now, having said all of that, the Wizard could avoid the cost of scribing and buying new spellbooks (and lugging a ton of spellbooks around) by purchasing a Blessed Book for 12,500 gp (no scribing cost and it has 1,000 pages--or equal to ten standard spellbooks).
And I think you are forgetting, Wizards need to have a back-up book at their disposal, in case their travelling book gets sundered or stolen or confiscated. They only get two free spells to scribe per level, so they are paying the full scribing costs for that backup. Remember too, that it takes 1 hour per spell level to scribe a single spell into a book (regular or backup or Blessed). Time is not kind of Wizards who want to add spells and craft.
If you look at all of this, I think you might come to realize that Wizards are already paying out the wazoo for additional spells in their books. If you are using the rules that are already there and in print.
Master Arminas