How do you counter-offer a potential promotion ?

I'm lucky enough to be considered for a new position, but I have to move to a different city with a much higher cost-of-life.

They offered me an 18% raise, but the area is approximately 20% higher than where I currently live. I want to let them know that I am only willing to move for a 30% raise.

The HR guy is pretty casual, but I'm not sure how to let him know what I want. I'm supposed to let him know if I take it tomorrow, so should I just call him and say "hey, I'll take the job IF I get xyz" ?

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