-In most cases, dual-casting deals roughly 10% more damage than single-casting twice. It definitely deals more damage than single-casting twice, but it's hard to be exact with the percentage, since you can't hurt yourself with your own spells (including Fire Rune, which won't go off if you or your companions walk over it, and even won't damage you (but will damage your companions) if you set it off with something like Flames).
This is comparable to the Conjuration dual-casting bonus, though, where dual-cast Conjuration spells last exactly 10% more than twice the single-cast duration, as determined by the effect's description on the Active Effects page.
-In all cases examined, dual-casting costs exactly 40% (well, with some rounding to whole numbers) more mana than single-casting twice.
-As your Destruction skill goes up, Destruction spell costs go down. For example, increasing Destruction from 21 to 100 lowered spell costs to 65%-70% of their original value. This is true for spells both covered and not covered by your mana cost-reducing perks, e.g. Novice Destruction.
-Magicka regeneration stops while "holding" a pre-cast spell (obvious) and while channeling spells like Flames (maybe not so obvious)
-Flame Cloak is a bugged exception: it still has 280% of the single-cast mana cost, but according to the Active Effects page gains nothing (i.e. neither damage nor duration) from being dual-cast.