You know, it all depends, professional stage managers - even AEA - work from $00.00 a week to over $2,000.00 a week, and, here's the kicker, it has little to do with your experience, it has to do what with the contract or the producer is willing to pay. You have too look at what realistically what can this producer afford to pay and what I am willing to do the job for. Sometimes those numbers are so far off, you need to walk away from the job, sometimes they are so close, it's not even worth negotiating.
Given you are willing to do it for free, but would like a little something, maybe something in the $50.00 a week range would be realistic . . . I know nothing about the scale, scope or production values going into the show - nor how much of your time you are willing to be there. Of, if it is going to more labor intensive, maybe asking for minimum wage - they are of of AEA contracts that don't work out to that rate. But, your email has few details. If you want to private me more details, I think I can customize your advice.
I would hesitate you pricing yourself OUT of the running, as you really seem to want to do it . . . and it maybe a good experience for you.
Best of luck,