♥ Pre-existing condition (Cardiac)

Hello.
How much extra cost it it for Long-QT without symptoms (a cardiac syndrome)?

My family and I are about to be relocated from the UK to California, and I have concerns regarding the medical insurance costs.
My wife has a relatively-common cardiac syndrome called "long QT syndrome". It doesn't affect her day-to-day life.
I wonder how much extra cost that will put on our medical insurance premiums, deductibles etc. (Private HMO plan)
I was told that the actual cost will be quoted only after the insurance company's physician will review her case.
Nevertheless I'm afraid that the extra cost might be so high that our entire relocation might not worth it. It puts a very high financial risk which I currently cannot estimate its scale: will it be an extra of $200/month (bearable) or $2,000/month (game-changer!)?
In addition will we be charged extra for being a new comers to the USA? If so - in what scale?
I do have a US employee, but he covers only a small portion of the premium ~$200.
Many thanks :)