Enchantments add new attributes to an existing item. Items that are already magical, or that already have an enchantment on them cannot take another enchantment.
Sane Magical Item Prices was used as a starting point for pricing, but ongoing adjustments are being made to the pricing, and additional items continue to be added to the lists.
DMG – Dungeons and Dragons Dungeon Master’s Guide
GGR – Guildmasters’ Guide to Ravnica
MOT – Mythic Odyssey of Theros
PHB – Dungeon and Dragons Players Handbook, 5e
TCE – Tasha’s Cauldron of Everything
XGE – Xanathar’s Guide to Everything