Most likely a main backflow is required at the main municipal meter, which normally would suffice for your whole property. Sub-metering with individual back flows normally cost less than $125 (3/4'' supply line) plus labor. However may not meet the requirements of your municipal supplier.
If freezing is not an issue raising the backflow device above ground, outside the parameter of your existing meter pit may be an economical option.
Also, note their are on going fees to test and certify the main municipal backflow device.
We’re here to answer your water sub-metering questions.
Southern Water Management