Here is a rough outline of all of the items in game, and their prices. Please note, these are only a guideline, and prices are subject to change, so don't follow this guide to the letter. Also, this guide shows items that can't be found in shops.
Rares -
Cracker - 1.5 Billion GP
Red Partyhat - 750 Million GP
Blue Partyhat - 1 Billion GP
Green Partyhat - 480 Million GP
White Partyhat - 500 Million GP
Purple Partyhat - 550 Million GP
Yellow Partyhat - 600 Million GP
Santa - 400 Million GP
Blue H'ween - 200 Million GP
Green H'ween - 200 Million GP
Red H'ween - 200 Million GP
God Armour -
Bandos Chest Plate - 50 Million GP
Bandos Tassets - 45 Million GP
Godswords -
Armadyl Godsword - 100 Million GP
Bandos Godsword - 50 Million GP
Saradomin Godsword - 45 Million GP
Zamorak Godsword - 40 Million GP
Saradomin Sword - 45 Million GP
Casket Rewards -
3RD Age Mele (Full Set) - 1 Billion GP
3RD Age Mage (Full Set) - 750 Million GP
3RD Age Range (Full Set) - 700 Million GP
Black (G) (Full Set) - 20 Million GP
Black (T) (Full Set) - 20 Million GP
Other Weapons -
Dragon Claws - 50 Million GP
Dark Bow - 25 Million GP
Rune C'Bow - 15 Million GP
Ancient Staff - 15 Million GP
Other Armour -
Dragonfire Shield - 60 Million GP
Dagon'hai Rob Set - 130 Million GP
Spirit Shield - 150 Million GP
Blessed Spirit Shield - 500 Million GP
Arcane Spirit Shield - 430 Million GP
Elysian Spirit Shield - 450 Million GP
Divine Spirit Shield - 455 Million GP
Spectral Spirit Shield - 460 Million GP
More coming soon... Please be patient.