I'm not sure I follow. +0 gets 1? And - doesn't have to be connected in any way... It's a separate trait? If it worked like that couldn't you curse someone with infinitely negative strength for free?
Strength +1 [2]
Strength -1 [0.5]
Having no modification doesn't change the cost, but lowering by 1 halves the cost, and raising by 1 doubles the cost. Therefore I say each point over 1 is double the cost.
Cursing someone that way is different. These traits count for creation, so you could give them lower strength to afford more power elsewhere (lots of magical power, but a frail body).
When cursing someone, lowering their stats is an advantage, and therefore cost more (it's a curse after all), so lowering their stats would cost more.
Cursing a strength 2 Agent to strength 1 would cost X, but lowering the same Agent to strength 0, aka. normal strength, would cost double X.
Of course, I think good effects make the curse cost less. So if you have a curse that makes the mind weak but strengthens the body, that wouldn't cost as much as just weakening the mind.
This could potentially be abused... if it wasn't for the game having a GM, and as such, it will only be a curse if it meets certain criteria.
Of course, I'm mostly just talking from how I would've done it, micelus is the GM, so I guess we'll have to wait and see what he has to say about it.