What is the average registration rates for websites with mandatory free registration?

They vary widely based more on name recognition and what is offered.
By and large, most mandatory registration based websites are free and money is made selling the compiled list of registrants as valid sales leads. If you're planning on implementing such a site, I very strongly recommend an opt-out option to build trust.
If you're not that well known, I'd probably be willing to pay about $0.99 a month (unless your content is pornographic, then I'd pay much more) anyway here's a couple of articles about how others approach the matter.

http://labnol.blogspot.com/2006/05/free-...

http://www.powerhomebiz.com/vol114/subsc...

http://www.nytimes.com/2007/09/18/busine...

I'm not wondering about how much to charge, the registration is always going to be free.

I need statistics on what the average registration rate is:

Example: For every x amount of visitors, about y% sign up for a mandatory free registration website

Ok, I understand now.
the answer leads back to what you're offering and because of that, I don't think you'll ever get a straight statistic.
for example, I found a website usridernews.com (a biker website) that has mandatory (free) registration. I would say that website has a MUCH lower registrants per visitor ratio than say www.area93.com (a local radio station in my city) which has better incentives for people to join.
Maybe this analogy will work, what's the ratio of window shoppers versus people entering a shop without windows. Well, it depends on what the store's known to sell. Some stores don't need windows, while some can't survive without them.
What I would do is try to narrow down what category your website falls under and then start searching for terms such as:
visitors versus subscribers to blogging sites

my philosophy on this is not to make it easier to visit another site for content that you want people to find on yours.

I was hoping to get someone who knows the statistics and can compile them here. Thanks for your help but perhaps somebody else knows the answer.

Hello.

Maybe: www.bravenet.com and www.alexa.com

Any doubt or another question, let me know it please.

Thank you very much for visiting me at Quomon.com