U.S. has not been released. Bimmer Magazine states that the UK pricing for the 328i convertible (325i in England) will be approximately 2,100 British Pounds more than the coupe. This means that the difference will be about $3,800 U.S.
However, the difference in the 335i pricing will be about 4,000 British pounds, which is near $7,000 difference.
For what it is worth, usually Bimmer Magazine gets it correct.