A "Decked" item just means that it's gotten all of the modifications that a player can add to an item to make it better, either give more armor or hit harder. You'll be able to see what modifications an item has by examining it. Here's a breakdown of all the things a decking requires:
Soulbinding is a Cleric spell that binds an item to the soul of the holder so they don't drop it when they die. It requires a moonstone, which costs 1000 gold, and all the caster's mana points, sometimes up to 400 points. So just the materials alone cost about 1800 gold. Some people will do "sb's" for free, but its not that common. Clerics can buy the soulbind spell at level 12, but it's not worth casting until AT LEAST level 16, if not higher, because fizzling costs you all that mana and the moonstone.
Enchanting is performed by Sorcerors and takes half of the casters mana. So for supplies, around 200 gold is needed. Level 24 and 25 Sorcerors/Enchanters can Bright enchant, which is the best any player can do. Enchant comes in four levels, Dull, Soft, Bright, and Brillant. The first three are based on a players level, the strength of the enchant increasing with the caster's level. Brilliantly enchanted items are usually given out by Staff as prizes in some events. Each level of enchant adds 4ac (armor count) to body armor, 2ac to helms and shields, and approximately 5-10 damage points to weapons. Prices for enchants vary greatly, sometimes as much as 5k for a bright enchant, if the caster doesn't really want to bother with it. You can often find someone who will just do it for the cost of mana, or sometimes free, so ask around some.
Blesses, like soulbinding, takes all of the caster's mana, and can be performed by lvl 20 or higher Clerics and lvl 23 or higher Paladins. This endows a blessing from the god of the caster's divinity. Each bless adds one ac to body armor. Shields and Helms cannot be blessed. Each divinity's blessing will allow a weapon to do greater damage against a follower of an opposing divinity. Again, prices vary depending on the person, but you can usually find someone who will bless for around 500 gold.
A fully decked item will have the following in it's description: The (item name) radiates with the power of fire, ice, earth, storm, and night. The (item name) is enchanted, and glows brightly. Embedded in the surface of the (item name) is a moonstone that glows softly.
If you do end up paying for an entire decking, it can cost you up to ten thousand gold. It's not always worth it to get a store-bought item fully decked. For example, while a plain staff costs 10 gold, a Magic Staff (or mstaff) is worlds better, and you can usually buy one already decked for around 30-50k ("k" meaning thousand gold, 50,000 gold). Jeweled Staves, of a jstaff, can go for a lot more...maybe 300-500k depending (these values tend to sway as new items are introduced, or demand increases).
If you need to prioritize these things, the one thing you should put at the top of your list is the soulbind. It's not worth spending any other gold if you drop the item the first time you die.