i think you should compare Bite and Penitence.
both abilities are somewhat similar, though there are important differences.
here's the chart for Penitence (source: http://demigod.wikia.com/wiki/Penitence)
cast time: .1 (nearly instant), range: 20 units, cooldown: 7 seconds, note: stuns for 0.1 seconds, will interrupt spells
rank 1 - 450 mana - 200 damage - speed reduced 7% - damage taken increased by 7% - 5 second duration
rank 2 - 550 mana - 400 damage - speed reduced 10% - damage taken increased by 10% - 6 second duration
rank 3 - 650 mana - 600 damage - speed reduced 13% - damage taken increased by 13% - 7second duration
rank 4 - 750 mana - 800 damage - speed reduced 16% - damage taken increased by 16% - 7 second duration
now here's the chart for Bite (source: http://demigod.wikia.com/wiki/Bite)
cast time: 0 (truly instant), range: 8 units, cooldown: 7 seconds, duration 3 seconds
rank 1 - 300 mana - steals 300 health - armor reduced 250 - speed reduced 15%
rank 2 - 420 mana - steals 500 health - armor reduced 400 - speed reduced 20%
rank 3 - 500 mana - steals 700 health - armor reduced 600 - speed reduced 25%
rank 4 - 595 mana - steals 900 health - armor reduced 800 - speed reduced 30%
the numbers say alot. i don't think my argument needs to be explained all that much further from here. you can plainly see that the two skills have alot of overlap in their effect but the damage and mana efficiency on Bite is radically higher than Penitence (particularly so if you consider health drain to be like double damage).
i know there are significant differences as well. Penitence is longer range. Penitence lasts longer. Penitence damage debuff applies to everything not just the stuff that is affected by armor. Penitence interrupts. all of these make the comparison not exact, but that doesn't detract from the main point.
either the damage or the mana cost of Bite is a bit wrong right now. thats the main point. and its pretty easy to see how wrong when you compare it to a very well balanced and popular and useful ability like Penitence.