How many grams of Saffron for Rice?
A quick search does not really give a good answer to your question

For example, one recipe calls for 2 pinch of saffron for 2 cups of (dry) rice; another 3/4 tea spoon of crumble saffron for 3 cups of rice.

Have a looksie at this wikihow page.

In any case, I would highly suggest you try to find either Spanish or Iranian saffron, and if possible not pre-grounded. 

And skip whatever cheap substitute you will find.