Informative post on playwright royalties can be found on the LinkedIn Theatre Producers group: http://www.linkedin.com/groupItem?view=&srchtype=discussedNews&gid=1781490&item=75996770&type=member&trk=eml-anet_dig-b_pd-ttl-cn&ut=0zMxCa6X4ril01
Unfortunately, you will have to be a member to read it. Since it's free, take the time to sign up or, if you are already a LinkedIn member, sign up for the group: Theatre Producers.

In the exchange of ideas and experiences on that post, you'll learn that a playwright who is a member of the Society of Authors (http://www.societyofauthors.net/) will get free legal advice regarding contracts beginning immediately upon joining the group. Associate Membership is available for those who are not published, self-published, or POD. If you're under the age of 35, yearly dues are around $100.00 which is a heck of a deal when it comes to getting legal advice that could keep you from being deprived of the money you deserve for the effort you put in to creating your play. BTW, the SOA has been around for over 125 years, having been founded by Alfred, Lord Tennyson.