actual poundage for resistance bands?

I was wondering how they catagorize the weights or resistance on most bands. I currently have dumbbells up to 25lbs. I need a way to get moreweight for my workouts. I do P90X and Im looking for a way to get more weights for my workouts.

Will a 30lbs resistance band give me 30lbs of resistance if I step in the middle of it and do curls or will it give me less?

can anyone give me some examples of yalls experiences witht he weights.