I just added a new meter in my barn that I had been servicing from my home meter for 20+ years.. For me, the math was easy, especially if you don't have any big up-front install costs.
I have Duke Energy and my monthly bill has a "customer charge" of $9.66. The energy costs (Per KWH) rates are stepped - 1st 1,000 KWH are .07633¢ plus an energy charge of .03698¢. That totals .1137¢/KWH.
Above 1000 KWH the rate is .09259¢ plus an energy charge of .04698¢ which totals .13957¢ per KWH.
The difference is .02587¢ per KWH x 1,000 KWH = $25.87 savings on the first 1,000 KWH on both meters. So when I was pulling from the house - basically, I didn't get that $25.87 benefit at the barn and the power was billed at the higher rate.
Plus - the sub panel in the barn was 600 feet away so I was paying for power at the house and I'm sure there was some fall-off along the way.
I think it depends on the power company charges/rates as well as distance. For me, I needed more power and the install costs were very reasonable. For some of the replies I saw to your post with bigger monthly meter charges, it might be tough to justify.