5 speed transmission kicks out of 1st gear. on 1996 BMW Z3

Happens all the time almost every time I try 1st gear.

Asked by for the 1996 BMW Z3
worn syncro in the trans. it will require removal and replace the syncro assembly.

How much would this cost? I have had same problem beginning at about 65000 miles or 40000 miles ago. Car runs ok but ready to sell and would prefer to have it working satisfactorily prior to sale.