First, an assistant is very important to a shooter, and an excellent assistant (they're rare) is something a good shooter will happily pay dearly for. But as has already been mentioned, a good assistant is often a damn good shooter in their own right, and could literally finish the job if the photog happened to be a no show. Furthermore, an assistant photog relationship is also something that develops over years. With this, pay becomes dependant on your skills, the place your relationship is at with the given photographer, and lastly the jobs overall budget.
IMO, in a market like Boston, starting rate for a 2nd or 3rd assistant would be $175. That's what I started at in a B market. Sometimes the shooters may have a little more and sometimes a little less, as long as you know their honest and just trying to get you on the money train too then it's usually good to be humble and accept the odd end $100 day. It often pays off in the long run if the photograpeh is generous, b/c they'll remember it when the big advertising jobs come along. Plus, it's a free lunch. (Fashion often pays less b/c there's more people in line to do it, and still life often pays more, b/c it's not as glam).
With that, if you get to know a shooter and become a 1st assist for them, learn them and their gear and how to predict them, learn to light for them, learn digi tech and retouching and file management (invaluable these days), and so on and so forth, then you should be making between $300-600 a day for commercial work. This may take a year or more to work to from the $175 starting point.
I assisted one guy for 2.5 years, we worked a lot and well together, and had a great relationship. I moved on, moved far away, and am doing little assisting now, but he still flys me home (east to west coast) and pays me generously to be his 1st when he gets big advertising jobs. I mention this b/c it illustrates how important a good assistant is, and once you start shooting big jobs on your own you'll understand why. They can make a shoot SO much better.
So become good, work hard, use your head, and you'll make good $ for yourself and a photographer. After which (2 years is the standard assisting time) you'll always have a close friend and mentor to call on with questions/favors/advice.