The 9305 is build using new silicon. It is a new chipset, only marginally related to the original 9300 series chips. It is, in theory, same to higher performance and lower power. Plus it natively supports up to 24 lanes per chip, where the 9300 was pretty much limited to 8 lanes and the 16 lane part was really two chips etched on the same die and tied together with a PCIe switch - and the really rare 24 port cards were really a 9300-8i + an LSI expander chip bolted together.
All that said - the 9305 ought to be the more expensive chip.
But in the server marketplace 'newer' is not always better. A lot of purchasing gets done based on what is proven and tested. So the 9300 series boards still have most of the market space. It's also not clear what driver bugs may exist for the new boards, etc. As the new boards get through various peoples qualification testing the market for 9300-8i/16i boards will shrink and prices will come down. The 9305 series boards should stay about where they are for now as they are basically selling at the manufacturers reccomended range.
Persaonally, I would really prefer the 9305 boards right now. 33% lower power for 16 ports - and 24 port version is selling for only about $100 more. If you have a 24 drive chassis getting the 9305-24i would be a much better approach than getting a 9300-8i + expander or 3x 9300 cards.