I don't think you're looking at this the right way. The right metric is value, not price.

If the community isn't worthwhile, then even $5 would be too much.

If the community helps your business to grow, then $50 is a trivial price.

I'm not a member, but I read Rob's book and it's was worth FAR more than the cover price.

I have revenue. Not as much as I want, but enough so that even a tiny increase in conversions is worth quite a bit, if the increase is permanent.