The Music Producers Guild never really got into recomending a standardized contract...it's just not The American Way. Producer managers typically get 15% of your fee and of any royalties.....of course if you are really hot it could be negotiated down to 10% or less.
And to Julien, yeah there are sharks, but good representation has helped many engineers, mixers and producers. When you are busy working, you need someone to field your calls, keep your name out there, negotiate your deals, see that you get paid (and not be taken advantage of), act as a production coordinator, etc. Sure, there are unscrupulous folks everywhere, but the right manager can help you mucho.