You can always go WGA on this one, or, at least, use them for a framework.  Pay for completion of a first draft and pay upon a second.  Get a piece of the back end, still.  We're wrapping up negotiations on a TV pilot and we got pay at signing, upon delivery of treatment, upon delivery of first draft, upon delivery of second draft, and all other rewrites are paid after that.  We also got a nice deal on the back end if the show moves into production.  At any rate, get your pay up front, but do it in a way that's fair to you and your client (that's what he is).  Make sure he sees results for money spent.  

Stokes, is this guy who asked you to turn his idea into a script, a friend of yours?  If not, why did he ask you...how did this all come up?

You can go 1 or a combination of 2 routes here...

You can ghost write it for him for a fee of whatever you charge for your time, or you can enter into a collaboration with him, where you each get credit and money if you guys can sell the script.

Quoted from Dreamscale Stokes, is this guy who asked you to turn his idea into a script, a friend of yours?  If not, why did he ask you...how did this all come up?

Friend of a friend kind of deal.  A friend of mine knew I wrote screenplays and threw my name in the mix when this guy was discussing his idea.

Gotcha.

Does he have the ability to pay cash up front?  It's really a matter him paying for your time, or you seeing so much potential in the project that you want to go into it with him as writing partners of some kind.

Hey Stokes.  I agree with Murphy in that this isn't a commission and that he's asking you to spec.  I have to ask: what does the other guy get out of it?  Is he attaching himself as producer?  Will you own all the rights to the script?  I assume you should since you are speccing it.

From what I know, typically writers spec for free for producers unless the project is already set up at a studio and there's money flowing i.e. studio-level assignments.  Producers get their money from the studios and unless they have discrentionary funds they can dip into (most don't), they can't pay you.  So, whenever producers have a cool idea for a project that they're sure they can sell, they pitch it to a writer whose work they like (based on sample spec), and if the writer likes it he'll go ahead and spec a treatment -- then the script.  But in the end, the writer owns the rights to the script since he wrote it.  But producers can get ugly if you suddenly decide to shop it elsewhere.

Not all producers have a Joel Silver status that can get a project set up at a studio prior to bringing a writer onboard.  And not all writers have the track record where they can sell on pitch.  Most producers need a script to show the studios -- hence getting writers to spec it first.

From what I know, if producers pay you upfront with discrentionary funds, then that's all a writer is typically going to get for the project, even if the script sells afterwards for much-much more.  Of course all of the WGA backend stuff will still apply if the project goes into development.  My understanding is that the basic mantra applies across the board: they don't get paid until you get paid -- from agents to managers to attorneys.  So it's in everybody's interest to have a strong script that can sell.

From what I know, there are a lot of spec sales by first-timers where the inception of the spec comes from boucing ideas with their manager or producers that have taken an interest in their writing talent.

The worst case scenario is that it doesn't sell but you've gained fans in the industry along the way, which opens up many more opprtunities.  This is how most writers typically build a reputation in town and kickstart a career.  You'll be surprised at how one little spec can attract the attention of heavyweights simply by getting passed around from a single source.  

From my understanding and experience, that is how the industry typically operates.