I like using Spell Research in my game and I think the experience is improved by making the available spell tomes less accessible to purchase.
I think jpieman55 did a great job with their mod, Really Expensive Spells, but the spells are more expensive than I like, so I used the magic of xEdit to change the pricing of the spell tomes to better suit the way I play.

You can pick and choose what you want installed via the new FOMOD.

The math for the new prices:

I multiplied the base value of each spell tome based on the school it belongs to, as follows:

-Restoration: All spell tomes are now x4 as expensive. If a spell tome originally cost 20 it will now cost 80.

-Destruction: All spell tomes are now x5 as expensive. If a spell tome originally cost 20 it will now cost 100.

-Alteration & Illusion: All spell tomes are now x6 as expensive. If a spell tome originally cost 20 it will now cost 120.

-Conjuration: All spell tomes are now x10 as expensive. If a spell tome originally cost 20 it will now cost 200.

What each plugin does:

-ValuableSpells Vanilla: Increases the prices of the spell tomes in SkyrimSE and its DLC (Hearthfires, Dawnguard, Dragonborn).
Note: While I did not personally test this with Skyrim 1.5.97 or below, there shouldn't be any issues. If there are, please feel free to leave a comment and/or make a compatible version.

-ValuableSpells Saints Seducers Vanilla: Increases the prices of the spell tomes in the Saints and Seducers CC (requires the Saints and Seducers CC, do not use alongside the EC version).

-ValuableSpells Saints Seducers EC: Increases the prices of the spell tomes in the Skyrim Extended Cut - Saints and Seducers mod and forwards the other changes made by it (requires the Saints and Seducers CC and Skyrim Extended Cut - Saints and Seducers, do not use alongside the Vanilla option).

-ValuableSpells Apocalypse: Increases the prices of the spell tomes priced above 0 in the Apocalypse mod (requires Apocalypse).
Note: There are a couple of spells in the mod that are priced at 0, I assume they were left this way for a reason and left them as they are.

-ValuableSpells Waterstride: Increases the price of the Waterstride spell tome in the Apocalypse - Waterstride mod (requires Apocalypse - Waterstride Spell Addon).

-ValuableSpells Odin: Increases the prices of the spell tomes in the spell tomes in the Odin mod (requires Odin).
Note: Odin changes some vanilla spells so I updated their price as well.

-ValuableSpells Phenderix Evolved: Increases the prices of the spell tomes priced above 0 in the Phenderix Magic Evolved mod (requires Phenderix Magic Evolved).
Note: Some spell tome descriptions did not match the spells they taught, so I also changed their description to match their effect.

-ValuableSpells Phenderix World: Increases the prices of the spell tomes priced above 0 in the Phenderix Magic World mod (requires Phenderix Magic World).
Note 1: Some spell tome descriptions did not match the spells they taught, so I also changed their description to match their effect.
Note 2: This mod adds new categories to spells, they currently have no weight on the price as I used the vanilla schools for the price calculation.

-ValuableSpells Mysticism_Vanilla: Increases the prices of the vanilla spell tomes altered by the Mysticism mod (requires Mysticism - A Magic Overhaul). Choose this if you only want the altered vanilla spell tomes to have their prices increased. Do not use alongside the Mysticism_Full option.

-ValuableSpells Mysticism_Full: Increases the prices of the spell tomes added or altered by the Mysticism mod (requires Mysticism - A Magic Overhaul). Choose this if you want both the altered vanilla spell tomes and the new spells added by Mysticism to have their prices increased. Do not use alongside the Mysticism_Vanilla option.

-ValuableSpells Mysticism_Jump: Increases the prices of the spell tomes added by the Mysticism - Jump Spell Addon (requires Mysticism - A Magic Overhaul and Mysticism - Jump Spell Addon).

-ValuableSpells Darenii: Increases the prices of the spell tomes added by the various spell packs by Darenii (requires Darenii's Spell Packs). Currently supports Abyss, Arcane, Arclight, Bloodmoon, Desecration, Flames of Coldharbour, Inquisition, Lunaris, Natura, Necrotic, Necrom, Plague, Stellaris, Vulcano.
Note 1: Necrom has 2 options for spell increasing - Restoration multiplier (x4) or Conjuration multiplier (x10). I made this because despite them being marked as Restoration tomes, the Daedric nature of Apocrypha themed spells could place them under "summoning daedra", hence maybe Conjuration being more fitting. I couldn't decide, so I made both, you pick whichever you want.
Note 2: I also changed the prices of the Power Tomes added by many of Darenii's mods, based on which school of magic I thought they would fit best.



NOTES:

I recommend that you use this mod alongside:
-Spell Research or a different mod that allows you to learn spells by studying instead of using spell tomes.
-Scarcity or a different mod that reduces the amount of items (particularly spell tomes) you loot.
-NPCs react to Necromancy and Invisibility, for immersion.
-Necromancy is Taboo for immersion (will need a patch for modded spells however).

Some of the plugins are newly created, which means they may not work with older Skyrim SE versions. You may need to use Backported Extended ESL Support to make them work.

These plugins will likely conflict with other mods that make changes to the spell tomes themselves. I will look into patches but it's going to take a while. Feel free to make your own patches if you want meanwhile, I don't mind.

My price changes were based strictly on the original prices and not on what they do/how powerful they are. This can lead to strange pricing logic (Example: The Bound Bow costs 3300 while the Bound Battleaxe costs 990). I may change the formulas in the future but I don't promise anything.

I may have miscalculated the price of a spell or two, I looked over them 3 times already but I can miss things. If you notice something odd please feel free to leave a comment.

I chose the current multiplier formulas thinking about how the schools are seen by Skyrim's population. Restoration is the most universally accepted, leading to higher supply, more open trading and so on, while conjuration is the most shunned, leading to scarcity and more secretive trading, which would bump the price.

Do keep in mind that based on your modlist, the prices that vendors will sell the spell tomes for may vary. I only changed their base value, I don't intend to change anything related to the vendors themselves.

I will likely not create an AiO anytime soon, if you want to create and share one you can.

CREDITS:

-EnaiSiaion for their Apocalypse - Magic of Skyrim mod + Waterstride Addon and for their Odin mod.
-ExtendedCut for their Skyrim Extended Cut - Saints and Seducers mod.
-phenderix for their Phenderix Magic Evolved and Phenderix Magic World mod.
-SimonMagus for their Mysticism - A Magic Overhaul mod and addons.
-Darenii for their large collection of spell mods.
-jpieman55 for the original Really Expensive Spells mod, which this mod is based off of.
-Bethesda for Skyrim and Saints and Seducers.