I think this has been discussed but IMHO the good SF troops they are looking for don't need bonuses to stay. I wouldn't turn it down but I never re-enlisted for no foogin bonus -- if there was one.
If soldiers with 10 years+ service are looking to get out, the problem isn't money. I speak from personal experience. I got out after 15 years and no amount of money would have changed my mind. I went in at the beginning of the VN war and my last ETS came after a period of uselessness. I guess it could be said that I didn't adapt to the peace time army. If the war had lasted three more years I would probably have re-enlisted and retired. I was offered a lot of things but the only thing I wanted was "OUT"!!
The incentives for continued employment progress from money for the newbies to training and job satisfaction and security as one progresses. It takes money sometimes to lure new recruits, but it takes more than money to retain good personnel. This is a perpetual problem in civilian life as well as military.