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

If you simply aggregating information than I don't see any issues with copyright. But, I would seek permission from the brokers/website owners to make sure business is protected. It doesn't hurt asking for permission, and if it directly benefits them to get leads/sales than none of them will say no.

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.

Obviously, no 2 situations are alike; and multiple factors affect any outcome. Practically, the number of answers is infinite. But one factor I've looked at intensively, full time for years is the role played by the brand name and/or the site's domain(s). Think of doing business -- online or off -- as moving along a path. Some paths are rocky or go through quicksand. Others can be made straight and smooth. Obstacles can be cleared, or the surface may be lubricated. For most niches, you'll see brand names / domains that add friction -- friction that is compensated for by extra marketing inputs ... effort or money. Suppose your niche were nicotine patches. Well, ideally you might own NicotinePatch(es).com to simplify brand recognition, add trust, increase click-through rates, and so forth. Traffic can be built up without an exact-match domain. No doubt about that. Still, not all domains / names perform equally well online or in the minds' of an audience. Answers aren't always so clear cut. However, since the internet is built on domain names, domains and names are worth evaluating very deliberately.

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.

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.