Definitely an interesting and popular idea, but in terms of practicality and data "freshness", very difficult to implement, especially at this stage.
Some of the issues that come to mind are the fact that there are so many different pricing schemes. Unlike gasoline which is charged by the gallon everywhere, with charging stations you have per kWh, per minute, tiered per minute, on/off-peak pricing, members-only pricing, free and discount programs (e.g. 3 years free charging at Electrify America; 1000 miles of free Supercharging from Tesla). Trying to capture all those variations in a concise and understandable fashion is definitely a task I'm not up to right now!
The other thing is that for many areas, it's not like there is a huge choice anyway. If you are on a road trip, you pretty much have to stop at such and such charging station, regardless of the cost. It's not like you exit the highway and can then shop around for the best deal among the 10 stations located at that exit.
That said, I think an interesting variation would be a simple $, $$, $$$, $$$$ rating. Subjective, yes, but I think it would be possible to assign a $ (free or almost free), $$$$ (clearly the site host doesn't want to encourage charging here--typical at car dealerships) rating to the extreme, and then be choose between $$ (below typical) and $$$ (above typical) to the remainder.
Expanding the idea even a bit more, maybe even have a 3 category rating system: size, cost, reliability
Probably a better idea for a site like Plugshare, but if they're not going to do such a thing, maybe I could try it here.