Reverse-engineering mass haste and mass suggestion, it seems as if changing a spell that affects a single creature to one that affects one creature/level, no two of which can be more than 30 ft. apart increases its level by 3.
Mass charm is 4 levels higher than charm monster, but it also loses a somatic component, effectively making it stilled (+1 level).
Mass invisibility is 5 levels higher than invisibility, but affects any number of creatures, no two of which can be more than 180 ft. apart. Hence, it does not really follow this template.
The only exception in the core rules is mass heal, which is two levels higher than heal. I believe there was a mass strength in one Dragon magazine, but I'm not sure if it followed this rule.
By this reasoning, mass hold should be 8th level, if hold monster is used as the base.
Actually, this suggests a new metamagic feat, Mass Spell, which will allow the spellcaster to modify a spell to affect one creature/level (all within 30 ft. of each other) at the cost of increasing the spell level by 3.