It's hard to tell exactly how much you will make from Runecrafting, all we can say is you should make a lot depending on what runes you make.
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You can train Runecrafting fairly fast at the RuneSpan. The entrance is at the Wizard Tower; the fastest way to get there is using the wicked hood's "teleport" option. The Runespan doesn't give you any money; nor does it cost money. Once you have a certain runecrafting level, you may want to make runes instead. This is slower than the Runespan activity, but you can earn money while you train.
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You can actually earn money, if you choose to do it the slow way (making runes). Especially with air runes you can earn a lot of money. However, if you want to do it faster, you can make tiaras (for example, combine air talisman and a tiara, to create an air tiara). This will cost money, but the amount of money varies, depending on what tiaras you make. Another option is the Great Orb Project, where you can actually gain money, too.
A skiller at combat level 3 can use a tiara. You can use any tiara, providing you have the Runecrafting level necessary to make the associated altar. Even without the level, you can still wear it.
Yes, its a really good way to make money.