Stabilizer: 65 - 75 mil
Catalyst: 15 - 20 mil
Neri G: 40 mil max
SJ: 6 mil
Mors: 100 mil
Letter A: 5-10 mil (I've heard that the archer Robbinhood sells it at 2 mil per A)
Letter H,L,O: Around 30 mil (??) (Will find out the best deals soon by sending shouts)
Letter W: 4 bil (??) (Will need to confirm)
Crystals: 15 - 25 mil
Storage Box: 15 mil
Box of 100 Neri L: 120 - 150 mil (if you have the patience to use 100 Neri Ls and Wz is a constraint then go for this instead of Neri Gs)
Opal, Garnet, Peridot: 3 mil each
Box of Rubies,Sapphires and Topazes: 80 mil per box
Saving scroll: 25 mil
UJ: Buy it from hatrel NPC (Buy a box of 100 UJs at 100 mil if you are buying in bulk)
LMS: Don't even try this

SJ: 3 mil
ESS: Each level beyond 100 costs you 1 - 2 bil (depends on seller)
Tart: 100 mil
Snow C, Stocking M : 130 - 150 mil
MOR (Mark of Rubal): 120 - 150 mil
Redyan's necklace: 45 bil
HK g8 sword stuff (mounted): Varies upto 100 mil per part
Grahsat level 10: 21 to 23 bil max
Grahsat level 15: 40 to 45 bil
Grace: 18 bil
Soul: 27 bil
Wide Darts scrolls: 13 bil
Bragir level 10: 12 bil
G9 part, weapon: 550 - 650 mil (Maxed Pants, Boots can be upto 1 bil)
Sunitz level 10 : 45 bil
Sunitz level 15: 75 bil
Arnir level 10: 45 bil
Arnir level 15: 60 bil
HK 9.5 mace level 10: 35 bil
HK 9.5 mace level 15: 50 bil
HK 9.5 blade level 10: 40 bil
HK 9.5 blade level 15: 50 bil
Warrior 9.5 spear level 10: 35 bil
Warrior 9.5 spear level 15: 56 bil
Warrior 9.5 axe level 10: 25 bil
Warrior 9.5 axe level 15: 40 bil
Warrior 9.5 sword level 10: 35 bil
Warrior 9.5 sword level 15: 48 bil
Mage 9.5 Paum level 10: 35 bil
Mage 9.5 Paum level 15: 50 bil
Mage 9.5 dex level 10: 40 bil
Mage 9.5 dex level 15: 50 bil
Unique Part: Costs aprrox. 41 bil per part ( Around 123 bil - for a complete set - Armor, Helmet and Gloves)
G10 Parts: Any part not more than 17 or max 18 bil
Helm, Gloves can be purchased at even as low as 12 - 13 bil sometimes.
HK g10 mace level 15: 60 - 70 bil (as quoted by Attila)
HK g10 blade level 15: To be verified
Correct me if I am wrong.
I'll add others later after verifying. This is to help people from other servers and who have no much idea about the prices here.
Remember, every deal depends on the no. of buyers and sellers, and their item/WZ urgency, but make sure you don't end up paying way too much.
Regards,
Razerwire