Everything costs money. This is a comprehensive guide to how much things cost in Runescape. These prices can fluctuate depending on area, resources, and if the seller hates your guts.
1gp - Full glass of clean water, a bronze nail, low-level rune
2gp - Plain beer, loaf of plain bread, small basic clay pot, ball of wool
5gp - Bowl of stew, fresh baker's bread, basic hammer, tea, basic sack
10gp - Cheap meat, most simple household items (pots, jugs), mid-level rune, bronze bar, oak log
20gp - Prime cut of beef, basic bronze tools, iron bar, fancy cakes, willow log, regular bow
30gp - Good glasswear, filling family meal, linen sheets, oak bow, maple log
40gp - Bottle of good wine/booze, most good iron tools (pickaxe, shears)
50gp - Bushel of grain, steel bar, silk sheets, high-level rune, piece of good tanned leather, rudimentary table or chair
80gp - Most good steel tools, small livestock, basic sword
100gp - Cheap full outfit, mid-sized livestock,
250gp - Large livestock, new custom-tailored fine outfit, simple wooden cart, mahogany log
500gp - Basic iron chainmail armor (full suit), specialty livestock, mahogany bow
500gp - Basic iron chainmail armor (full suit), specialty livestock
800gp - Basic covered wagon, just under peasant's monthly income
1k - Well-made iron platemail armor, just over peasant's monthly income, small blue dragonleather item (one piece)
2k - Basic steel platemail armor (full suit), simple diamond pendant, typical emerald ring, cheap yew log
3k - Simple mithril sword, adamant bar, nice diamond ring
5k - Ornate mithril greatsword, low noble's monthly income
8k - Barn, shed, little empty workshop, multi-diamond necklace, full green dragonhide outfit
10k - Basic small house, peasant's approximate yearly income, full blue dragonhide outfit
15k - Nice small house, well-made adamant longsword
18k - Working class citizen's yearly income
20k - Basic family-sized house, basic mithril chainmail armor (full suit)
25k - Nice family-sized house, well-made/ornate mithril platemail armor (full suit), full red dragonhide outfit
30k - Onyx ring or pendant
40k - Fleet of five mid-sized ships with one large flagship, full black dragonhide outfit
50k - Mansion, just under low noble's yearly income, magic log
80k - Slightly more than a rune bar
100k - Well-made adamant platemail (full suit), magic bow
200k - A small town's reserve/emergency treasury, opulent mansion, fortified castle
250k - A very wealthy noble's yearly income
300k - Basic rune sword
500k - A poor small town's main treasury
1MIL - A flourishing small town's main treasury
2MIL - Slightly less than rune platemail (full suit)
5MIL - A mediocre large city's main treasury
10MIL - A flourishing capital city's main treasury
Pack Animal/Mount Animal - 250gp
New basic tailored outfit - 100gp
New formal gown/suit - 250gp
Silver bar - 345gp
Gold bar - 575gp
Full set of bronze plate armor / Sword / Bronze bar - 400gp / 60gp / 10gp
Full set of iron plate armor / Sword / Iron bar - 800gp / 80gp / 20gp
Full set of steel plate armor / Sword / Steel bar - 2k / 100gp / 50gp
Full set of mithril plate armor / Sword / Mithril bar - 23k / 3k / 750gp
Full set of adamant plate armor / Sword / Adamant bar - 90k / 12k / 3k
Full set of rune plate armor / Sword / Rune bar - 2.25MIL / 300k / 75k
Dragon metal weapon - Priceless
Small basic house - 10k
Mansion - 50k
Small town's main treasury - 500k-1MIL
Major city's main treasury - 5-10MIL
Working peasant's monthly / yearly income - 900gp / 11k
Working classman's monthly / yearly income - 1.5k / 18.5k
Low noble's monthly / yearly income - 4.75k / 57k
High noble's monthly / yearly income - 22k / 265k
Teletab (for a non-mage) - 25gp
Wooden ships: small / average / large flagship - 4k / 6.4k / 8.8k
Average sapphire uncut / cut / ring - 80gp / 160gp / 1.3k
Average emerald uncut / cut / ring - 360gp / 720gp / 1.9k
Average ruby uncut / cut / ring - 600gp / 1.2k / 2.3k
Average diamond uncut / cut / ring - 750gp / 1.5k / 2.7k
Green dragonhide piece / tanned&sewn / full armor - 432gp / 865gp / 7.8k
Blue dragonhide piece / tanned&sewn / full armor - 575gp / 1.1k / 9.9k
Red dragonhide piece / tanned&sewn / full armor - 1.4k / 2.8k / 25.4k
Black dragonhide piece / tanned&sewn / full armor - 2.3k / 4.5k / 40.5k
Royal dragonleather - Priceless c'mon now
Regular log / bow - 04gp / 20gp
Oak log / bow - 10gp / 24gp
Willow log / bow - 20gp / 44gp
Maple log / bow - 30gp / 64gp
Teak log / bow - 135gp / 275gp
Mahogany log / bow - 250gp / 500gp
Yew log / bow - 2.1k / 4.2k
Magic wood log / bow - 51.5k / 103k
Elder wood log / bow - Priceless