Michael D. makes a good point - adding submetering will definitely increase resale value, I would guess you'd get all (or most) your money back. Although this may not be a significant deciding factor if you plan on holding for 10-15 years. Also consider this, with my particular water co. (in northeast PA) - if submetered, the water bill is attached to the tenant. There is no lien on the property if a bill goes unpaid. The water co. will go after the tenant for payment. Not sure if this varies with each water co. or if this is by county, check with your local water co. With the RUBS system, if a tenant vacates with an unpaid water bill - you must pay it (then try to collect from tenant in court, etc).
I think Joel said it already - it sounds like you may be too late, but depending how close you are to closing, ask the seller to pay for it, or pay a comparable amount in closing costs. K. Marie Poe has some good advice - if you're interested, it's fairly easy to get details of utility billing from the owner or utility companies, then do your own analysis.