The way charging is billed varies by state because of different utility regulations. For some, it's time based. For some, it's power based. In CO, it's priced per kWh directly.
Here's a little bit of information on the time-based costs: https://www.tesla.com/support/supercharger#payment. Looks like I might be able to look up the cost of an individual Supercharger via the car's nav system. You can't get it via the website that I've found, but you can see all of them: Find a supercharger. That link also shows Tesla destination chargers, which are usually free but are a Level 2(ish) charging speed so they're for when you're stopped for something else. Staying in a hotel with a destination charger (Tesla or otherwise) is fun because it's like someone comes by to fill your gas tank for you at night and you don't have to make a gas stop before you hit the road.
A Model 3 will do, very roughly, between 3 to 4 miles per kWh. I use the former for my calculations even though our lifetime is quite a bit closer to the latter. So 60 kW is approximately good for 180 miles.
Your pot-smoking friend would have been better to unplug at 80%, the charging rate drops off pretty quickly above that. Of that 20 minute stop, he probably spent a third of it getting that last 9%. But then he wouldn't have been able to get quite so relaxed, man.
Our least expensive charging stop this week was also the only one where we got over 180 kW, I think I saw 220-something briefly. A Model 3 can only do that when it's fairly close to empty, the new S can hold that speed much longer. Charging rates are like horsepower, everyone only pays attention to the peak number but it's area under the curve that really matters.
It was interesting how we found out we no longer got free Supercharging. On our first stop this trip, I plugged in and walked away (lunchtime!). The car proceeded to charge but I eventually got an alert that Supercharging was suspended until I entered a payment method. I was able to easily approve the card that Tesla has on file (for the premium data subscription or for ordering parts) but it was interesting that Tesla didn't automatically help itself to that payment method. The car didn't stop Supercharging, it just wouldn't have allowed me to do it again. It appears that Tesla errs on the side of "let the car charge and we'll figure the rest of it out later".
I know that's true in the case of a communications error - if the Supercharger can't make contact with home base to confirm access, it'll just turn on. Most of the other networks do not. There was a recent survey of CCS charging stations in the SF Bay area that showed that around 25% of them were not working, and the most common problem by far was payment authorization failures.