This guide will give you a good idea of what an items worth.
I'll try and keep this updated if prices raise/fall substantially.
These ranges are *not* set in stone.

Prices can vary, it's always in your best interest to check the Auction House to see if something has raised/lowered since this was last updated.

Combat items:

Weapons:
Rune 2h: 45-55k
Rune Battleaxe: 30-40k
Dragon Sword: 100-120k
Dragon Axe:

Armor:
Addy Full: 0-30k
Large Rune Helmet: 30-35k
Rune Platebody: 50-70k
Rune Chainbody: 35-40k
Rune Plate Legs/Skirt: 50-60k
Rune Kiteshield: 50-65k
Dragon med helm: 200-300k
Dragon Sq. Shield (Right half): 750k-1m
Dragon Sq. Shield (Full): 1m-1.2m

Prayer:
Dragon Bones: 7k-9k each

Magic:
Air Rune: 10-20gp each
Water Rune: 8-11gp each
Earth Rune: 8-11gp each
Fire Rune: 10-20gp each
Death Rune: 150-200gp each
Blood Rune: 200-250gp each
Law Rune: 100-150gp each
Nature Rune: 125-150gp each

Ranged:
Bronze arrows: 15-25gp each
Iron arrows: 30-35gp each
Steel arrows: 40-50gp each
Mithril arrows: 75-80gp each
Adamant arrows: 120-150gp each
Rune arrows: 300-500gp each
Longbow: 0-10k
Magic Longbow: 0-20k

Misc
Ruby amulet of Strength: 1k - 3k each -- noted drops
Diamond amulet of Power: 1k - 3k each -- noted drops
Dragonstone amulet: 17.5k - 20k each
Charged Dragonstone amulet: 18k - 22k each

Skilling items:

Herblaw:
Filled vials: 100-200gp each
Guam: 500gp each -- hard to sell, only for beginning herblawists
Tarromin: 800gp each -- not as valuable with str pot drops from mobs
Ranarr: 2-4k each -- always in demand
Irit Leaf: 3-5k each -- always in demand
Kwuarm: 4-6k each -- always in demand
Cadantine: 4-7k each -- always in demand
Dwarf Weed: 5-8k each -- will be in demand if p2p wild becomes more active
Snape grass: 500gp - 1k each -- noted drop & ground drops (hobgoblin isle)
Eye of Newt: 500gp - 1k each -- noted drop & store bought
Limpwurt root: 2-4k each -- noted drop
Whiteberries: 3-5k each -- noted drop & ground drops (red dragons)
Wine of Zamorak: 2-4k each -- easily obtainable, just time and click consuming (chaos temple)
Strength potion: 300gp - 750gp each -- depends on f2p pking, noted drops of 100 (black demon)
Prayer restore potion -- 5k - 7k ea -- noted drops
Super Attack potion -- 5k - 8k ea -- noted drops
Super Strength potion -- 5k - 9k ea -- noted drops
Super Defense potion -- 5k -- 10k ea -- noted drops
Super set - 15k - 30k ea
Ranging potion -- 5k - 10k ea -- will be bought/sold more with p2p wild activity

Fishing/Cooking:
Raw Swordfish: 500gp - 750gp ea
Raw Shark: 500gp - 750gp ea
Cooked Swordfish: 750gp - 1k each -- currently best fishing moneymaker due to f2p wild activity
Cooked Shark: 750gp - 1k each -- will be more widely bought/sold with p2p wild activity

Woodcut/Fletching/Firemaking:
Yew logs: 500gp - 750gp each
Magic logs: 700gp - 1k each
Arrow shafts: 100gp - 150gp each
Headless arrows: 150gp - 200gp each
Feathers: 20gp - 100gp each -- depends on demand. Large quantity drop via greater demons
Bowstrings: 400gp - 1k each -- depends on demand (only useful for people training fletching)
Steel arrow heads: 25gp - 35gp each -- large quantity drops (greater demon)
Mith arrow heads: 40gp - 50gp each -- large quantity drops (greater demon)
Addy arrow heads: 75gp - 150gp each -- medium quantity drops (greater demon)
Rune arrow heads: 250gp - 350gp each -- small quantity drops (greater demon)

Mining/Smithing/Crafting:
Iron ore: 100gp - 150gp each
Coal: 600gp - 800gp each -- large quantity noted drops
Mithril ore: 650gp - 1k each -- medium quantity noted drops
Adamantite ore: 3k - 5k each -- small quantity noted drops
Runite ore: 10k - 15k each -- small quantity noted drops
Steel bar: 500gp - 900gp each -- large quantity noted drops
Mith bar: 1k - 1.5k each
Addy bar: 4k - 6k each
Rune bar: 18k - 23k each
Gold bar: 500gp - 1k each -- small quantity noted drops
Flax: 100 - 200gp each -- hard to sell

Runecrafting:
TBD

Holiday rare items:

Yellow Partyhat:
Red Partyhat:
Green Partyhat:
White Partyhat:
Blue Partyhat:
Pink Partyhat:
Christmas cracker:
Christmas Cracker:
Santa Hat:
Elf Hat:
Reindeer hat:
Grinch mask:
Santa Top:
Santa Bottom:

Green Halloween mask:
Red Halloween mask:
Blue Halloween mask:
Scythe:
Pumpkin:

Bunny ears:
Bunny suit top:
Bunny suit bottom:

Half full jug of wine:
Disk of returning: