A balancing factor for the wizard is THE SPELLBOOK.
The spellbooks means that spells beyond his FREE ones take away both time and money from thw wizard. Net result he has less time and wealth to turn into magic items, either by making them himself or by buying them.
Comparably, a wizard at 6th level who has scribed 3 additional spells at each level 1st-3rd has spent 3600 gold and 27 days. His compatriot, a bard, has those 27 days and 3600 gold to turn into items by feats or purchase or whatever.
That serves to offset the wizard's extra spells somewhat.
Often i think of the wizards extra spells as if they were magic items, just ones with no experience cost. Like a fetish which allows you to prepare a spell not on your list for instance.
Now lets drop this to 1 gp and we see the wizard now has little reason other than time to not scribe every spell he can into his spellbook. Unless this is a campaign where time matters he does not seriously have to choose between SPELLS or ITEMS but can have it all.
They are meant to make the wizard have to make a choice, item or new spell, and dropping them drastically will eliminate this choice, just increasing the power.
This will show up severely when the wizard loses a spellbook. His equipment like the fighter's sword or armor is subject to loss too and costs to replace. A fighter losing a +2 sword is at risk for about 8k+, which is about the same as a full spellbook (10k).
If the full spellbook costs only 100 gp to replace, suddenly the wizard is making out much much better.
My recommendation... if your campaigns money overall if off the baseline...
check your PCs loot and wealth totals. Figure out the average. Compare it to the average expected PC wealth tables in the DMG. If you plan to keep that ratio of loot, then adjust the cost of spells appropriately.This should offset any campaign skews while still keeping the costs in balance.
But, IMO and IMX, just dropping the cost altogether or making it trivial will simply up the power of the wiz and remove a very significant balancing feature.