I tend towards fairly dense nonfiction like Scale (finished yesterday). That's nearly 20 hours at 1x playback, which the effective price was $15 w/ my membership.

I find a huge amount of value for the effective $1-2 per hour I end up paying, even before considering the no-thought-required return policy and pro-rating the cost from multiple listens.

YMMV, but I consider my audible subscription some of the best value I receive for my money.