TeslaFi and probably a few other API-based Tesla data trackers will tell you exactly how much it costs. Also keep in mind that 120v charging is also dramatically less efficient at charging than 240v.
120 Volt Charging is necessarily less efficient., there is just much less amperage available and therefore the Charge Cycle will be much longer. If you have a 10 watt bulb in a lamp and you burn it 100 hours you have used one kilowatt of power. If you have a 100 watt bulb in a lamp and you run it 10 hours you will have used 1 kilowatt of power. If you put 50 kilowatts in to a Telsa, if it takes 5 hours or 50 hours for the charge, it is still 50 kilowatts which in my area is about $5.50 USD. Some Chargers on the road by the minute. But at home you pay per kilowatt...
The absolute max energy that you could draw (if charging 100% of the time) is 1,000 kWh or $100 at $0.1kwh. Now you know that you cannot charge 100% of the time - unless you are just using the car as a heater, you'll need to drive it. A good rule of thumb would be 50% driving, 50% charging - that would be a max of $50 (for unlimited usage). The actual math is harder to do since it takes longer to charge than it does to drive. Now, if you are just going to the store, your usage won't be trivial since you still need to run the A/C each time, but you are probably looking at between $10-$30 a month at 10 cents per kWh.
I live in Ohio, I have been driving 100 miles per week watching my grandson. I keep the heat set at 71 degrees and tend to like to see how fast I can get to the speed limit between lights. My Model Y is a blast to drive. Driving wild and crazy I am in the low 300's (Kw) per miles. I took a 1,200 mile trip Thanksgiving weekend it was 27 degrees during my last leg.. My last leg was 199 miles Lexington, KY Supercharger to my home in Worthington, Ohio. I left Lexington at a 90% SOC (State of Charge) and arrived in Worthington, Ohio at a 19% SOC. That is 71% to go 200 miles, With a 74 Kilowatt battery that mean that I used 52.54 Kilowatts, with the Heat on 71 degrees and driving 70 mph. At my local rate of $0.1233/kWh that means that is cost me $6.48 to drive those 199 miles or about $0.0325/miles. Three and a quarter pennies per miles at highway speeds with the heat on... If you really take it easy you can go a long way on very little money....
Yes, measuring by time will tell you the amount of power drawn from the wall, but predicting the charge time based on battery pack size is a very coarse estimate, especially when charging at 120v. Efficiency is only 60-70%, and that's without the car having to run the battery heater.
I think there’s a mistake in your Model S calculation. Assuming full battery rating (and not usable capacity), you had Model Y at 74000/316. By the same token, Model S would be 100000/390 = 256 Wh/m and not 390 Wh/m. Overall your assumption is correct and, on paper, the Model Y should be cheaper to operate and recharge, 234 vs 256 Wh/m, but the difference isn’t as large and may even favor the Model S in real-world conditions. I don’t know if EPA testing factors in wind resistance or it the car is tested inside a building. Tires can also make a difference and affect efficiency. I recall an article stating the S doesn’t lose as much efficiency at higher speed compared to the 3/Y. As such, depending on the type of driving, you may use less energy in an S. At 65 MPH, my Raven S was averaging 230 Wh/m (better than its EPA rating):