I'd take the $15 and be pleased. What was your alternative?
I paid ~$35 in Study Butte, $20 at Stillwell, $20 at Hueco Tanks, $20 (voluntarily) at The Stable. I expect the cost of electricity would be about $5 so a charge of $10 would be fair. However, I want these places eager to see EVs.
Incidentally, the guy at the The Stable seems interested in putting in destination chargers. I explained he could put in two units that share a 100 amp breaker. I suggested he charge a fee in some way. My preference is just a sign "voluntary contributions via PayPal to <emailaddress>". I could generate no interest in a destination charger at the Holland but I did not see a good site for it. The Stable does have a good site between his two buildings. I burned half a day of daylight charging at The Stable because I found the 14-50s locked when I pulled in after dark Sunday night and could not start charging until ~9am Monday; I pulled into Alpine with about 12 miles which went down to about 8 while I slept at the Holland. I suggested to the guy that there was little reason to keep the outlets locked. A better Alpine charge opportunity might be the one or more RV parks to the east of campus; there are several walking distance hotels.
In Iraan a couple of months ago, I tried the Mexican place nearly adjacent and thought I could do better. More recently, I attempted the Mexican place across the highway, closed on Sunday. Remind us of the location of the BBQ?