how much damage for magical backlash?

[Thac0 Redeye]

My players will soon enter an area that is in a zone of magical disruptive caused by the BBEG. When they cast a spell they will have to roll a 20 die. on a 1 the spell will fail, on a 20 the spell will be empowered (like the meta magic feat) and on a 10 the spell will go off normally but the caster will take damage from magical backlash. This will also include wands and scrolls so i want it to be based on the spell level and not caster level. My first thought was 1d6 damage, but that's only approx 3 hp per spell level. i want it to be more than a mosquito bite but not game breaking. As they get closer the field will get stronger. Eventually the rolll will be on 1,2-9,10-19,20 and near the end they will roll a 1,2,3 -9,10,11-18,19,20. They will be around level 10 or 11 when they first enter zone and be around level 18 or 19 near the end. ( this will also effect the bad guys too)
Any thoughts?

Thanks

[Biggus]

Given that full casters mostly get their new spell levels two class levels apart, 2d6 per spell level would keep it in line with the damage most spells do. Possibly 1d6 for a 1st-level spell and then 2d6 per level.

[Beni-Kujaku]

Most of the time, magical backlash doesn't result in damage, but in rounds dazed (Celerity, Chosen of Deneir, Born of Three Thunders...). It's the price to pay for abusing something you shouldn't. Not a real problem outside of combat, but devastating in threatening situations. I think something like 1d6 per spell level plus one round dazed (or one round dazed per 2 spell levels at higher level of disruption) would be good.

[Vaern]

I wouldn't go for more than 1d6 per spell level. Any more and you're looking at damage comparable to being hit with an actual offensive spell. I'd tack on non-damaging penalties like dazed, fatigued, or sickened.
Or, apply some wild magic effects instead of just damaging backlash. The spell affects you instead of the intended target, or an unintended target instead of yourself if you are the target. The spell appears to fizzle when cast, only for the effect to manifest a few rounds later and operate as you originally intended. The spell has some effect directly contrary to its original function (darkness becomes light, cure becomes inflict, fireball deals cold damage, haste becomes slow, etc).

[loky1109]

1d6 per CL, if you want damage.