How to change standard rate for a resource at project Level?

Hi,

I have a resource whose rates vary depending upon his location . So how i can i change his rate in the same project lets just say he is charged with some 50$ from 1st to 15th and then 100$ from 15th to 30th or so on ... how can i achieve that.

I tried going into the resource sheet view and then under cost rate table to change the value and apply one efffective date for it, but as soon as i publish the project the changes that i make are lost !!! and it again takes back the default values set in the PWA for the resource , is there any way to achieve what i am trying to do here ??

Thanks