What is the cheapest way to make a mc server?

The cheapest way is completely free;
host your server off of your pc.
I personally only recommend this method if you want your server temporarily online and only to play with friends etc...

If you want an actual server then you're gunna have to get a host. Hosting companies like mcprohosting are overpriced so also not really a +1 for this method.
This method is good for regular faction servers or towny servers, nothing big, no hub,...

If you want to have a proper server, nvm, not server, a network then you should consider getting yourself a dedicated server, from ovh as an example.
These are pricey but they are worth the money. I think the cheapest one is 60 euros from ovh. You can host anything on here; mc servers, teamspeaks, websites,...
But either you will have to learn how to access the data (putty, winscp,...) or you're gunna have to get yourself a System-Admin that is experienced.
This method is good for networks such as: hcf servers (hub, and possibly multiple hcf servers,...)