Joking aside, I recall reading about a prototype helmet that uses solar power to operate a cooling fan in the summer and a heater in winter, but currently it is nothing more than that, and may never reach market.
I think the best bet, until something like that does become a practical reality, is to save your money on something that will make a marginal difference at best, and wear a balaclava under your current helmet. I just don't believe that there are currently any helmets out there, that are significantly warmer than another brand, to justify spending the money on one just to try to keep a little warmer in the winter. Invest a lot less in a quality balaclava, would be my advice.