$300 million sounds low to me, too, when they were talking about $500 million to build in the railyards. I'm sure both of those quotes are for arena/parking only and does not include all of the infrastructure costs.
Labor and material costs have come down, but I find it hard to imagine it reduced potential costs that much. Not only that, but by the time they could actually break ground on an arena, I expect some of those costs will be back higher. Not likely as high as they were, but not as low as they have been the past year or so.
Building in Natomas would likely be the cheapest option as there would be little to no land cost, depending on the deal, and the most important infrastructure is already in place. For other reasons I'd rather see dowtown or CalExpo where the arena would be more integrated into the neighborhood, rather than being out on an asphalt island as in Natomas.
I just hope we don't end up with another super cheap arena (like Arco), with little to no architectural appeal and can't be upgraded in the future. Palace of Auburn Hills was built the same year and cost twice what Arco did. Guess what? It was much niver to begin with and was built in a fashion that allowed it to be upgraded, which has already been done.