I've got a player who wants to either make a runestaff using Craft Staff or buy a custom runestaff and I cannot for the life of me figure out how the costs made in the runestaff section came to be. The 3.5 DMG has details for crafting a regular 50-charge staff, and a discount if you want to make it so activating a spell costs double charges to reduce the cost. Nothing for the "as spell used" activation type. Neither of the two mechanics in the DMG add up to the numbers given in the Runestaff section nor in the Magic Item Compendium for that matter. What is the actual crafting equation for Runestaffs?
DMG 3.5:
Highest level spell = 375 gp x caster level x spell level
Next highest spell = .75(375 gp) x caster level x spell level
X number of spells left = .25(X)(375 gp) x caster level x spell level(s)
Caster level must be the same for all spells in the staff and minimum caster level of 8 according to DMG 3.5. If using the mechanic for double charge cost, multiple all above equations by 1/2.