My experience with AC-21 and immigration: Big lawyers are less responsive.
If you are like me, who has a simple AC-21 case, has an urge to know when things happen to their GC process, and want a responsive lawyer, you would be better off using a local lawyer, even if they are not the most experienced. You can compliment that by doing self research on this and similar sites. That way, you would have the best of both worlds.. That is what I wish I had done...
However, if you want a name brand lawyer firm, Khanna, Murthy, Shusterman, etc are great.
Typical fees for informing CIS abt AC-21, applying for EAD/AP renewals, and answering any RFEs is between $500-$1000 in total. An additional fee of $500 or so would be required if you need to do FOIA to get labor job description. Make sure you agree on a fixed fee and don't get screwed on an hourly charge basis bullshit some lawyers try to lure you into.
Goodluck!
-ab