Specify payment

I've come across projects that state "$$$ per project" but it's really not clear enough. I was given a long script just to find out payment wouldn't be given until the writer finished the next one in an unknown time frame. Fast forward 2 years later and I never got full payment.
"Per project" also gives way for creators to eternally use someone's skill for 5 bucks, since it was technically "5$ per project" and the project happens to be a 5 year commitment with dozens of scripts or needs before the talent gets paid.