Starfire glass is low iron, so it is much more optically clear than standard glass. But like you said, the trade-off is a much higher price.
Fro what limited reading I have done, Iron (Fe) is added to the silica mixture for a couple of reasons. First is it helps lower the melting point thus improving the manufacturing process. Second, I believe it is added so the glass will reflect or block, UV rays. Most glass these days is only about 75% silica. The rest is additives to improve manufacturing.