WotC has not put out a metamagic feat called "Mass Spell" or, to my knowledge, a spell called "Mass Harm." There is another harm spell, I believe in Heroes of Horror?, but it isn't a mass spell.
Furthermore, just because Mass Heal exists, does not necessarily mean Mass Harm should as well. From my perspective, it would be too powerful. It's an arguable point, but not necessarily a popular one. Take that for what you will.
The general consensus from those that have houseruled a "Mass Spell" feat, is that it would require a spell level adjustment of at least +4. Truth is, depending upon the spell, it's such a highly variable thing that the closest metamagic feat is the much more limited "chain spell" feat.
I could see a new edition having each spell labeled with metamagic adjustments particular to each spell. It could be much more time consuming to design spells for such a system however and would require all metamagic feats to be implemented at the beginning of creation or have really substantial web support as new metamagic feats came out in supplements.