Does anyone knows how much cost to build a membership site?

You appear to have a very broad concept. The more features you add in the more expensive it will be. I'd recommend figuring out the key feature you think will appeal to potential users and build a MVP based off that. If that feature doesn't attract users, having the other features wouldn't have helped. I'd be happy to talk through your idea and make some recommendations building a test platform.

I have been managing multiple domains/sites for my clients since a few years now and I think the easiest and best way to do it in your case would be setting up simple WordPress sites on each one of them and managing them with something like ManageWP(https://managewp.com/) . If they're niche domains, you can monetize them through ads or use them as affiliate sites. Selling them can also be also an option. If you can get the sites to look fairly nice and if they are niche domains, you can sell them on sites like Flippa(https://flippa.com/) . Overall, you have many options. I'd be glad to help you in detail through a call if you want.

2 different solutions: #1: Ad Proxy They serve ads throughout the web and these websites act as referrals in order to gather statistics about the Ads. #2: Fake Referrals The websites use bots in order to fake referrals and appear on analytics tools like Google Analytics or Mixpanel. Either way, if you want to generate traffic, you'll have to rely on more high-level channels like SEO, PPC or Social.

Two things that immediately come to mind would be use of webinars, that allow for audience participation, covering a specific topic of interest to your audience for either a small attendance fee or as part of a premium subscription. I would lean more towards the premium subscription for the recurring income. The second would be to make regular book and audio resource recommendations from Amazon, providing your Amazon affiliate link to pick up the resource.

Most of my friends use www.wordpress.org and http://member.wishlistproducts.com/ to create their membership sites. Hope that helps.