It depends, are there a lot of people who start jobs but then give up after a couple of months? If the incentive is to keep people in the job for the year (by which time they might have got over the desire to go back to not working) then maybe.

When I used to work in a factory lots of people gave up after one week - one week isn't really long enough to get used to something. People who were there for several months tended to stay there, make friends get used to the money etc so maybe that incentive at the end of the year could make people think twice about quitting at beginning, and sticking it out for a decent amount of time.

Depends on the figures and rational behind it - I would want to see why they thought this would work before giving final verdict - (alot of policies are a result of working parties etc, analysis of figures etc. rather than just plucked from thin air)

I think costing wise would probably be negligible cost - when you factor into tax paid, other allowances might not get, super being gained.