Runecoins are type of RuneScape currency bought by microtransaction, used to buy cosmetic items in Solomon's General Store, found in the Extras tab (formerly Squeal of Fortune tab). They were released on July 17 2012.
They can be obtained in two ways, when players first visit the store they can receive 200 runecoins for free. But afterwards they have to pay with real world currencies to get more. On the second Jagex Runescape Livestream it was discussed that they were thinking of other ways to give players Runecoins other then 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 Soloman'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 three months from purchase. If Runecoins remain in a player's balance for three months after they were purchased or generated, they disappear without a refund. This applies to both the 200 free runecoins and purchased runecoins. 
|Currency||200 Runecoins||400 Runecoins (+20 bonus)||
800 Runecoins (+100 bonus)
|2000 Runecoins (+400 bonus)||4000 Runecoins (+1200 bonus)|
|New Zealand Dollars||$5.99||$11.99||$23.99||$59.99||$120.00|
Certain quantities of Runecoins are only available with certain payment methods.
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.
It mostly doesn't fulfil the conditions for RWT because the Runestore items are cosmetic only and have no real effect on the game, with the exception of extra bank spaces.