A few other differences - you don't interface with developers, price is guaranteed, costs are lower because dev talent isn't in house (no holding cost).
That is literally how it works at many of them, just fyi. I'd strongly, strongly recommend you hire someone who does PM for one of those companies if you are as oblivious to how they work as that statement just made it appear.
> A few other differences - you don't interface with developers,
Yeah, plenty of them just want you to talk to the PM. The only time I was ever brought into meetings was when it was a very, very large customer [e.g. Bigger than Zynga] and the contract was 7 figures.
> price is guaranteed,
Prices are often fixed in outsourcing contracts at a per-project price with a list of deliverables and features.
> costs are lower because dev talent isn't in house (no holding cost).
I've worked with outsourcing companies where they paid per-project or per-hour a specific project [usually projects under $20k] as their sole model.
I worked with 2-3 at a time and just kept the apprised of my availability, one usually was the bread and butter while the other two handed me small projects [usually 80 hours or less].
Prepayment isn't an option because you'd need to use something other than credit cards -- the chargeback rate would be far too high for any payment processor to keep you as a client.