I work with either advance payments (through SafePay invoicing) or escrow payments (with SafePay escrow). For larger projects, it's sometimes a combination of both.
For feature screenplays, a common payment template is an upfront payment of one-third, which covers initial research, the treatment, and the first act. A second payment covers the second act, while the third and final payment covers the remainder of the screenplay and a revision based upon your feedback.
Cost for screenplays and other creative services vary with subject and complexity, and I'm happy to answer questions and provide more information on any of my bids.
I believe in the value of frequent communication and am always available via email or Guru private message. Skype calls can also be arranged.