I’ve got the information for this list from average prices of potions, enchanted plushies and essences in trades. This is an approximate list and it is as accurate as I found the prices to be.
Rare costumes etc will of course normally cost a little more.
Last updated: 13/09/2022.

Arinya - 7mil - 10mil
Astro - 5mil - 8mil
Basil - 14mil - 20mil
Chibs - 40mil - 45mil
Crikey - 20mil - 30mil
Daisy - 15mil - 18mil
Dakota - 7mil
Decadal - 300ml+
Echlin - 4mil - 6mil
Ercuw - 12mil - 15mil
Eyru - 6mil - 8mil
Figaro - 10mil - 15mil
Gizmo - 5mil - 6mil
Gobble - 20mil - 25mil
Hump - 8mil - 10mil
Huthiq - 6.5mil
Ike - 7mil - 10mil
Justin - 50mil
Kronk - 7mil - 8mil
Kujo - 6.5mil - 7mil
Lati - 9mil
Limax - 30mil - 50mil
Lorius - 10mil - 12mil
Mordo - 20mil
Nino - 10mil
Oglue - 7mil - 8mil
Phanty - 5mil - 7mil
Poera - 6mil - 8mil
Pucu - 20mil - 25mil
Quell - 17.5mil - 20mil
Raulf - 30mil - 35mil
Rofling - 70-85mil
Rusty - 14mil - 15mil
Sindi - 13.5mil - 20mil (a lot of variation in prices)
Snookle - 12.5mil - 25mil (a lot of variation in prices)
Straya - 29mil - 33mil
Sybri - 13mil - 15mil
Tasi - 7mil - 15mil
Troit - 4mil - 5mil
Ushunda - Available in shop search (under 500,000mp)
Viotto - 6.5mil - 8mil
Vixen - 15mil - 30mil
Vlad - 18mil - 21mil
Wallop - 40mil - 50mil
Willa - 20mil - 25mil
Yuni - 20mil - 23mil
Zoink - 4mil - 7mil
Zola - 120mil - 130mil
Zoosh - 4mil - 5mil

Thank you to @ev, @meatbun and @parpy for your edit suggestions!