I looked it up on the "internet" and I was a little low. I found numbers from 10 lbs to 13.5 lbs is about equal to a 1 hp increase.
I've seen a ratio that drag racer use as a rule of thumb but I can't find a solid source. I did find several hp calculators based on ETs, trap and weight but not the formulas behind the calculator. Fun to play with regardless.