I think this is a valuable thing to know about your boat because it can assist in planning in a variety of ways.
We're in year 12 of being on our trawler nearly full time. We used to anchor about 50% of the time - that's down to about 10-25% these days because of our need to visit marinas. I put all the costs together for anchoring about 6-7 years ago and figured that it cost us about $25/day to anchor. Almost all of that cost is generator cost although I did allocate some for ground tackle depreciation. If we just anchor overnight, the cost is minimal since we rarely run our generator in that case. But when we're anchored for more than a night, the $25/day figure has been pretty consistent.
So having that data makes marina go/no-go decisions easier at times. $25 * 30 days is $750. So a marina monthly rate of about $700 becomes a no-brainer decision assuming it's a place we want to stay for a month (with $50 left for electric power consumption).
Even if a marina has a higher monthly rate, knowing this info makes the decision a lot easier. Let's say the monthly rate is $23/foot which isn't a low rate. For us, that turns into $1,219 + electric. But in reality, it's costing only $1,219 - $750 = $469 plus electric for the full month since we're not going to get away from that $750/month cost in general, anyway.