Honestly I think the key to success is acting like its an expense and monitoring it. I use to do that with heroin for the most part to avoid running out extremely early and being broke. After all there are other costs then just drugs regardless of how small they may end up being.
I have always treated my use as a monthly expense. As of lately I've been scaling down my purchases of pills and have been much happier with my bank account.
As for the original question, it would seem to be very dependent on the persons level of use, prices in their area, and purity.
I do an easy calculation of how much of my pay will go into buying drugs and I'm guessing meth addicts do the same..take 100%, move the % over to the left 2 spots and make it a decimal so 1.0 ..multiply that by all the money I will make and boom drug/meth expense