You get 5000 PLN gross under a project - or sometimes in Biology it can go up to 6500 PLN gross. But generally having PhD under project gives you a better salary than the regular one.
Cons
The university will see the the project-based PhD salary as gross-gross, they will take some part and resulting in 4300 gross before deducting 11% for mandatory social insurance. Actually not cons, but just how it work.