I have a couple of keyword.com domains that are niche specific that convert to paid membership at around 1:3000. Pays $30 per sign up (sell like 1-2 a month). I don't know what the industry average is but I'd guess it isn't that great as it once was.
Also, many providers have cut their prices as well as giving access to their entire catalogue of sites as an incentive for people to sign up.
If your keyword is just porn, I don't think I'd bother unless you have a real product you can ship to them or that is in demand. You just can't compete with all the tube sites that offer free porn these days.
I'd also avoid .com.au for running a porn site due to stupid censorship laws (no water sports, hentai etc) and the fact it's something I think you want to attack from a global instead of local perspective (unless you have a product to ship).