Making money is fast and simple! Here are some ways!
A common Money making way is Thieving. it is fast, and always making you money!
Another way is to kill dragons and sell there bones, which can vary on the seller or the buyer of how much cash you get!
Bossing is a dangerous activity ( in wilderness ) , Bossing can be fun, but dangerous at the same moment, at times, you might get a drop and you might not ( depends on drop rate ).
And The last MOST common way is to PK, if you risk fight, and you kill the other player, you can easily make money from it!