Well, while you are right about that, there is zero good reason to charge per gun as I see it. Charging a daily fee, or a fee for a 1/2 day, would make more sense to me. A daily fee was what I was used to up north. Seems to me they are discouraging, purposefully or not, how many guns anyone brings to the range. I used to often bring at least half a dozen guns with me to outdoor ranges. I'd not be doing that had they been charging a per gun fee, in fact I'd have been doing what the OP is doing - looking for another range.
What would you consider to be a reasonable fee for say half a day?