I'm interested in making a homunculus, but they require mandrake root to make them, and I can't find the price of it anywhere, so does anyone have any idea of how much it costs?
Mandrake is a common plant of no particular value
Mandrake is a common flower (in many parts of Europe).
There is no indication that this material component should be any pricier, or harder to find, than the other materials in the list. Heck, it’s in the middle of a list of common materials. Compare it to mistletoe, the material component of the Goodberry spell.
Mandrake root has been attributed with mystical powers in superstitions and various fantasy stories. But there’s no indication in the D&D rules that it should be less common than it is in the real world.
Xanathar’s Guide, Create Homunculus spell
Xanathar’s Guide to Everything includes Create Homonculus as a spell, which lists mandrake root as a component with no gold-piece value. Mandrake root is listed alongside clay and ash. Contrariwise, in a separate clause, another component is listed with a gold-piece value.
The rest of the answer is older material, which might still be useful if your DM doesn’t use Xanather’s Guide to Everything
The Monster Manual provides general guidance on how to create a homunculus, without providing an actual mechanic.
If the DM determines there is a regular spell to create the homunculus, then RAW, “a character can use a component pouch or spellcasting focus...in place of the components” (PH, p. 203: Components).
There is no Official Source
It's not priced anywhere in the PHB, MM, or DMG, meaning that it would be completely up to your DM.
If your DM is feeling generous, they may say that a component pouch includes it. Alternatively they might have you forage for it.
Your DM may Still Disallow it
MM pg. 188
Shaping a mixture of clay, ash, mandrake root, and blood, one can channel rare ritual magic to create a faithful, squirrel-sized companion.
Your DM may rule that you do not have access to the 'rare ritual magic'.
Essentially, as this is part of the MM, not the PHB, it is well within DM purview. Your first course of action should be to speak to them about it.