How Much WOuld It Cost To Build The Death Star?

This is presumably ignoring the battery capable of storing on the order of 10^32 kJ[1]. The steel is really the least of your worries.

[1] http://scienceblogs.com/builtonfacts/2009/02/the_physics_of_...

A link to a link to the actual source:

http://www.centives.net/S/2012/how-much-would-it-cost-to-bui...