I guess it will depend on where you are, but we just paid several thousands of dollars to have this done for our recently acquired duplex. We considered going the sub-metering route, which would have been cheaper. However, this is not allowed where we are. We had to pay the local borough for the installation of a new curb stop, piping, etc. And we had to pay a contractor do the excavation work. It may not be worth the cost to you, but we are planning on holding this property for the long term and the cost of doing the work would have been our outflow for water payments over a few years (had we continued to pay water as the previous owners did). We did the work now so we can have tenants assume the cost for water, pass on the administrative burden of billing the tenants, be be able to shut off water for any non-paying tenant without impacting the other, and also to make it a more attractive buy for future investors if we eventually decide to sell.
Anyway, in terms of actual advice - contact your local municipality/water supplier. They will be able to tell you if sub-metering is permissible. If it is, you contact a few providers to get cost. If it's not, they should be able to give you a quote for the work required to split the water between the units. Be sure to ask them if there are things they will not do and you need either a plumber and/or a contractor for (as stated, my borough would not do the actual excavation, but once digging was done, they did the underground work). Then, if you have to contract anything out, call a few contractors and/or plumbers and tell them what you need done and they will give you a quote for their portion of the work.
