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I am not the one that says that ink is too much. Quite the contrary. 15k gold for buying a 9th level spell means that the player can save 120k with his 9th level free spell knowledge. And one player would cringe at the cost for the ink? AND you only buy if you have no other choice. More often than not, a quest, a service or simple exchange spell for spell is made for additional spells. It pays to be kind with other wizards.
And strangely, almost the enterity of all wizards that played at my table were quite ok with this rule. Satisfied enough that the great majority of the casters are wizards and not warlock or sorcerer. I guess that my tax on versatility is not big enough...
On second reading, it seems you are confusing spell cost and ink cost. These are unrelated in my games. You pay the spell, then you pay for the ink to write it down. The free spells you gain each level are just that, free spells. You still have to copy them though. This is not a big cost when you think about it (unless your DM is starving you of money, which I am not).