Average cost per pad is typically around $200-400, depending on labor rates and the type of meter you get. It also depends on if your getting the new meters which can be read remotely, or if you still want meters that need to be read by either the manager or you. You could either buy them yourself and have a plumber install them, or you can higher it out and that will include the price of the parts as well. Direct read are around $70-$100 per meter, and remote are probably around $100 to $150 more than these.
There are a few companies out there doing this. I would go with the more expensive remote readers, because its one less item your manager/greeter has to do. especially on a smaller park like this, everything counts when it comes to the budget. Your manager may only be part time anyways and might not have time to do the reading, and then you have to invoice the water/sewer bills each month. Or it may be worth doing if you feel like you want more hands on experience or live close by.
Some people budget for about $500 a pad just to be safe.
Check city laws, but submetering water is normally okay by terms of the city, as long as its public water and not a private system. You mainly just want to pass thru the cost, don't up price the tenant for usage/labor cost to read the meters etc. This is where you can get into trouble.
Also, i would budget for only collecting about 75%-80% of the total water/sewer bill because like rent, tenants don't always pay their water as well.