Here is a tentative "Ballpark" price guide for BrettScape.
Santa Hats - 8-10b (depending on rarity)
Party Hats - 10-13b (depending on rarity)
Masks - 6-7b (Depending on rarity)
Ags - 1.6-1.8b
Sgs - 700-800m
Bgs - 400-600m
Zgs - 600-700m
Abyssal whip - 40-50m
Fury - 60-70m
DFS - 100-120m
D Bones - 2-4m each
Ahrims set - 100-120m
Dharoks set - 180-200m
Karils set - 100-110m
Torags set - 100-120m
Verac set - 140-160m
Guthans set - 100-120m
Bandos tassies - 170-180m
Bandos chestplate - 200-215m
Bandos Scim - 80-90m
Bandos Kite - 150-160m
Armadyl plate - 1.5-1.7b
Armadyl lets - 1.5-1.7b
Vesta Longsword - 2b
D claws - 3b
Vesta Spear - 1.8-2b
Stat warhammer - 2-2.3b
Stat Helm - 1.2-1.3b
All God armors - 100-125m a piece
Ranger boots - 25-30m
Robin hat - 40-45m
Dragon c'bow - 400-500m
Warrior, seers, archer, berserker rings - 150-250m each
Void - 100m a piece
Super sets - 20-40k each
Prayer Pots - 50-100k each
Death cape - 1.7-1.8b
Tri-gs - 4-5b
Fighter Torso and hat - 30-50m
Rune defender - 10-15m
Dragon Gs - 4-6b
Crystal shield - 20-30m
Untrimmed capes - 200-250m
Crystal bow - 20-30m
G maul - 2-4m
Herb Seeds - 5-10k each
Herbs - 5-10k each