RuneCoins

Runecoins are type of RuneScape currency released on July 17 2012. Bought through microtransaction, they are used to buy cosmetic items in Solomon's General Store, which is found in the Extras tab.

Runecoins can be obtained with real world currencies. On the second Jagex Runescape Livestream it was discussed that they were thinking of other ways to give players Runecoins other than buying them.

Items you can buy from Solomon's General Store include cosmetic outfits, titles, achievement banners, new animations for fishing, woodcutting, herblore, mining, and slayer, some unique emotes and teleportation animations as well as a new high alchemy animation. Additional bank space may also be purchased.

In the Solomon's Store FAQ, Jagex confirmed that Runecoins can only be bought through the billing system, and are not available through gameplay. It was also stated that Loyalty points, Squeal of Fortune spins and Gold cannot be used to buy anything from Solomon's General Store.

Runecoins have a maximum life span of twelve months from purchase. If Runecoins remain in a player's balance for twelve months after they were purchased or generated, they disappear without a refund.

Costs
Certain quantities of Runecoins are only available with certain payment methods.

Other offers
There are also packages available to American players. If an American player uses game cards (available for $10 or $25) they have a range of options when redeemed, such as spins:
 * 460 RuneCoins and 40 Spins for $25

Note:  These prices are only for online billing.

Game cards offer lower values than these. My flacid doodle looks like a little wiggly worm.

Premier Club
If a player joins the Premier Club and purchases a silver or gold package, the player will be granted 500 RuneCoins or 2000 RuneCoins respectively as part of their package.

The value of RuneCoins included in the Silver package is worth aprox $11.90USD with numerous other benefits included.

The value of RuneCoins included in the Gold package is worth aprox $41.70USD with numerous other benefits included.

Controversy
Runecoins have caused some controversy, as some players think that it is a form of real world trading (RWT) which Jagex has fought against for years. However, due to the nature of their terms and conditions of RuneScape, it is clear that RWT is defined as "a third party selling content of which does not belong to them". By this, this means that all items, XP and coins are property of Jagex, and so 'breaking their own rules' has no conclusive argument.