Just install another instance on another droplet.

For example:

apt-get install mysql-server

Make sure you bind it on 0.0.0.0 or your droplet's ip instead of 127.0.0.1 (/etc/mysql/my.cnf) too.

Will traffic between droplets will be counted in 1Tb (5$ droplet) ?

cyberprank, what was the answer for the datatransfer question.. I want to set up a mysql replica on a remote system, aka, master in us and the other droplet in am, will I be charged for inter network data transfer?

Well, my question is similar:

Can I setup my WWW droplet to comunicate with my MySQL droplet over LAN? My interdroplet traffic would no count for the Datatransfer cap, would it?

Well, since I found the answer to my question, let's share:

In this post (https://www.digitalocean.com/community/questions/additional-ip-addresses-internal-network-ip-addresses) the DO employee Raiyu says that private IPs is next on the list. Apparently the initial ETA was delayed a little, but I hope we can hear about it soon.