Details

Go to the Roundtable Hold and go to the NPC known as Twin Maiden Husks (the one that sells you the good stuff)

Item Name | Equip ID | Price
Runic Arc | 190 | 70,000 runes
Larval Tear | 8185 | 50,000 runes
Exalted Flesh | 1210 | 3500 runes
Pickled Turtle Neck | 1100 | 4500 runes
Gold-Pickled Fowl Foot | 1200 | 5500 runes
Silver-Pickle Fowl Foot | 1190 | 2750 runes
Golden Rune [9] | 2908 | 3,800 Runes 
Golden Rune [10] | 2909 | 5,000 Runes
Golden Rune [11] | 2910 | 6,200 Runes
Golden Rune [12] | 2911 | 7,500 Runes
Golden Rune [13] | 2912 | 10,000 Runes
Numen's Rune | 2913 | 12,500 Runes
Hero's Rune [1] | 2914 | 15,000 Runes
Hero's Rune [2] | 2915 | 20,000 Runes
Hero's Rune [3] | 2916 | 25,000 Runes
Hero's Rune [4] | 2917 | 30,000 Runes
Hero's Rune [5] | 2918 | 35,000 Runes

Installation

WARNING: Do not use online as it's not worth the risk.

1. Disable EAC > Anti-cheat Toggler
2. Make a backup of your regulation.bin.  Default location: c:\Steam\steamapps\common\ELDEN RING\Game
3. Drop my regulation.bin where the original is and overwrite it.

I'd also recommend backing up your save as frequently as possible; the "tool" I use backups every steam game I've played so far + other: Ludusavi Steam Save Tool

How to mess/add support

1. Download Yapped Rune Bear
2. When you're on the open section in the menu, select "Elden Ring" from the dropdown
3. Open the regulation.bin in the main game directory. Example: C:\SteamLibrary\steamapps\common\ELDEN RING\Game
4. Go to ShopLineupParam; they'll be named 
5. Value = price

This is all thanks to Xylozi  as I message him asking how he updates so quick because I'm relatively new to this, and tutorials on how to mess with it are scarce, and he replied with this masterpiece.

Important: Use the Import/Export Data function in Yapped to quickly merge the changes from the latest regulation.bin and my mod. I'll outline the steps you'll need to do to update a mod between regulation versions:
 

1. Load the vanilla regulation.bin in Yapped.
2. Change the Project Name in the Yapped settings to "Vanilla".
3. Click "Mass Export Data" and let it export all the params to CSV. Wait for it to finish.
4. Load your mod's regulation.bin
5. Change the Project Name to your mod's name.
6. Click "Mass Export Data". Wait for it to finish.

 
You will now have two folders in res/GR/Data, one called "Vanilla" and the other with your mod's name. You can then use a program like WinMerge or K-Diff to check the differences between CSV files relatively quickly. In WinMerge you can then sync any changes from vanilla into your mod's CSV files.
 
After doing that, you can load your mod's regulation.bin in Yapped, make sure the Project Name is your mod's name, and click "Mass Import Data". This will now update the regulation.bin with the merged data from the CSV files. If not much has changed (i.e. only a few params), you can click the Param you want to update and instead use "Import Data". This will import data just for that param, so it is quicker than the mass import.

Other Information

I downloaded Purchasable Larval Tears by liberator369 to see how he did it and so far it has worked out well (I hope).Thank you for naming everything to make this easier on me. I appreciate it. Moreover, he deleted his mod so I've added support to mine for the tears.

Other. If you want a mod that lets you "craft"/get a lot of items in your item crafting section (get items for free anywhere), then checkout Grand Merchant Mod.
My mod is similar to his, but instead, you buy items from the maiden at a set price that I feel is balanced. I can only do a certain amount of items because of how they've done it in Elden Ring.

Mods I recommend and or use:

Fix the camera
Faster Respawn
Unlock the framerate
Simple Vibrant ReShade for Elden Ring - No AC3 Remaster level yellow tint
Torrent to Kaiden - Horse Replacement
EMU Light - Enhanced Moveset Utility