I have only charger via Superchargers (free for my 2018 Model X) or at home. So I am ignorant of what would be considered a reasonable fee to use a Level 2 charger NEMA 14/50. It looks like some have a flat fee and others charge by the kWh. I was recently looking at an off the beaten path Level 2 charger location that was charging .60 per kWh. Does that seem high? I'm guessing there is really no standard for what to charge, seems they are all over the place.
Your terminology is a little confusing, but I get what you are asking (NEMA 14-50 refers to an electrical outlet, and while if you plugged in your mobile connector to it it would create a level 2 charging station, usually the L2 charging stations you find that charge money are not NEMA 14-50 outlets).
As for the cost...the cost can range anywhere from free to ridiculous. There is definitely no "standard", and it rarely relates to what electricity rates are (although in areas with high electricity costs, it's natural to assume that charging costs are going to be higher). Some hosts probably don't even realize what the costs really are and simply equate it to how much they are used to paying for gas. And some (car dealerships who aren't interested in charging stations at all but were forced to install some by their affiliated manufacturer) charge outrageous fees to use their charging stations because they don't actually want anyone using it.
But something I've found helpful in determining whether a cost is "fair" or not is to not think about it in terms of the electricity you are getting, but rather the convenience you are getting, and whether that convenience is worth it to you.
If you are out in the middle of nowhere with no fast charging nearby, and your only options are plugging into a 120V outlet for 3 days so you can make it back to the nearest Supercharger, or paying $0.60/kWh for an L2 charger that will do the job overnight, which are you going to pick? Well, if you are just sitting around camping and not really doing anything anyway, the outlet might be the right choice. But if you are on a road trip and need to leave the next day, I think the option is clear. And if you don't really need the charge anyway and were just wondering if you should take advantage of the L2 charger that's sitting next door, well, I guess you can make that call.