I wasn't able to get GoodRX to do what I wanted.
However I just discovered via the Humana stuff that came yesterday (new plan card, and other other info) and the help of a friendly rep:
If you input the Humana plan name and your zip code, a list of pharmacies shows up. Not the best format for the job but better than anything else I know ......it lists the pharmacies by a combo of distance/alphabetical...........
there is detailed info about each pharmacy. About the 2nd/3rd line down,
it tells you whether it is standard cost sharing/preferred cost sharing/out-of-network.
I didn't spend a lot of time searching.....but the only preferred cost sharing I saw for the Humana Walmart Value plan was Walmart. I'm assuming that all the preferred cost sharing sites have the same price but haven't checked that.