Trapdoor Costs and Locations:

Whiterun:
Cost - Free, automatically appears once Breezehome is acquired.
Location - The trapdoor is located under the staircase.

Riften:
Cost - 2000 gold
Location - The trapdoor is located under the staircase.

Markarth:
Cost - 2000 gold
Location - The trapdoor is located immediately to the right after passing through the entrance hallway.

Windhelm:
Cost - 3000 gold
Location - The trapdoor is located in a small nook to the left of the entrance.

Solitude:
Cost - 4000 gold
Location - The trapdoor is located on the basement level, in a short hallway to the right after descending the staircase.

Solstheim:
Cost - Free, automatically appears once Severin Manor is acquired.
Location - To the left when facing the player bed, behind the pillar.



Items Sorted by this mod:

Amethyst
Black Soul Gem (Empty)
Black Soul Gem (Grand)
Bonehawk Feathers
Bonehawk Skull
Chaurus Chitin
Chitin Plate
Common Soul Gem (Common)
Common Soul Gem (Empty)
Corundum Ingot
Corundum Ore
Daedra Heart
Diamond
Dragonbone
Dragonscales
Dwarven Metal Ingot
Ebony Ingot
Ebony Ore
Ectoplasm
Emerald
Firewood
Flawless Amethyst
Flawless Diamond
Flawless Emerald
Flawless Garnet
Flawless Ruby
Flawless Sapphire
Garnet
Gold Ingot
Gold Ore
Grand Soul Gem (Empty)
Grand Soul Gem (Grand)
Greater Soul Gem (Empty)
Greater Soul Gem (Greater)
Iron Ingot
Iron Ore
Leather
Leather Strips
Lesser Soul Gem (Empty)
Lesser Soul Gem (Lesser)
Malachite Ore
Moonstone Ore
Netch Jelly
Netch Leather
Orichalcum Ingot
Orichalcum Ore
Petty Soul Gem (Empty)
Petty Soul Gem (Petty)
Quicksilver Ingot
Quicksilver Ore
Refined Malachite
Refined Moonstone
Ruby
Sapphire
Shellbug Chitin
Silver Ingot
Silver Ore
Stalhrim Ore
Steel Ingot
Void Salts