AltGr + 4

Ubuntu is the first OS which supports Indian Rupee Symbol, Since release Of Ubuntu 10.10. Here is the one fact that nobody can type Indian Rupee Symbol without support of font. So font should have Indian Rupee Symbol in it. By default Ubuntu 10.10 is coming with that. So, it will make easy for your OS to display Rupee Symbol and copy paste of it, but typing of it is tricky, you can use Ctrl + Shift u2b09. But it would very hard for anybody to type frequently with above key combination. Also, it is very tricky add key combination with some hack. But now onwards it would be de-facto .