Gen 1: Nugget, sold for 5000 in Pokémarts, and mail at 25
Gen 2: Nothing that trumps the Nugget, so Slowpoke Tail and Star Piece come in at 4900, but Metal Powder and BrightPowder can be sold for 5.
Gen 3: No new valuable items that could be sold were introduced. Nothing trumps the Powders, but the Lucky Punch can be sold for 5.
Gen 4: Rare Bone ties with Nugget at 5000. All the items from before, but Quick Powder shows up, also with 5.
Gen 5: I'll handle this one differently.
- Pokémarts: Nugget and Rare Bone at 5000, and shoal shell and salt for 10
- Route 5 maid: BalmMushroom for 25000, any berries not listed here for 20: Lansat, Starf, Enigma, Micle, Custap, Jaboca, Rowap, Razz, Bluk, Nanab, Wepear, Pinap, Cornn, Magost, Rabuta, Nomel, Spelon, Pamtre, Watmel, Durin and Belue.
- Icirrus City man in Pokemon Center: Comet Shard for 60000, and I can't find a cheapest but I have sold gems for 50.
- Undella Town man: Relic Crown for 300000, and Relic Copper for 1000.
Red/Blue: You can't get any lower than 1... Apple, Big Apple, Grimy Food, Chestnut, and Plain Seed.
Explorers of Time/Darkness/Sky: Everything in the previous list, + Nectar.
There are no items in any of the games!!!