cost for the server 5.7.1

As it states in the article you've referred your post to, Server.App is now discontinued. I'm thinking unless you purchased it when it was available it is now not available for purchase from the App Store. If you did purchase it previously then you should be able to download it. Log into App Store using your AppleID account and access it via your Purchases tab. IIRC it used cost £19.99 (UK) or $19.99 (US).

If you did not purchase it previously and you wish to purchase it now (and unless someone else posts who knows of a way?) I don't think you'll be able to? However you could click the "Support" link at the top of this page and see if that gets you somewhere. Who knows it might?

Beyond your question I should point out that what Apple mistakenly calls a Server is not actually a server in the way most/all IT admins would recognise as a server (no SSO, no DHCP, no DNS, no LDAP, no VPN etc, etc. All that's left that is vaguely usable (in a limited way) is Profile Manager. PM in and of itself should really be viewed as a 'baseline' to the more functional and robust MDMs offered by non-Apple providers (JAMP, Mostly, Intune etc). It was only ever meant to be a PoC offering showing what could be done.

Hope this helps?