sub metering water on an 8 plex

[Caleb Clay]

I have an 8 plex that I pay the water on every month. It is a C or C- neighborhood with $550 gross rent per unit. What would it cost me to sub meter it? Is there a cheap and effective way to accomplish getting my tenants to pay their own water? I think I heard on one podcast that you can cut a hole in the sheetrock inside the unit and apply some type of small unit to get this done but cant find the podcast. Water bill is roughly $400 per month and would increase my NOI significantly. Any help would be appreciated! TIA