Not sure what you mean by "trunk bandwidth", it could be either backhaul or transit.
Backhaul (from customer to ISP POP) needs to match closely or exceed end customer bandwidth. Most last mile wholesale providers will offer a 1:1 or low contention SLA, although some "manage" the bandwidth (they oversubscribe until someone complains). However, if an ISP can't get this, or they build it themselves, they need to make sure your backhaul interconnects are overbuilt enough to deal with expected growth in line with however long it would take to build additional capacity. This is the primary cost of end user connectivity. The local loop price is usually bundled in with backhaul for costing.
Transit is cheaper and much easier to manage, these are your interconnects to other networks. It needs to be overbuilt as well to exceed peak usage by enough margin to allow continual upgrades, but aggregate usage across an ISP is generally much lower than the sum of its customers. Transit physical interconnects are usually delivered directly into an ISP's POP (a location commonly shared with other network users and ISPs, like a regional datacentre) and benefit from economy of scale.
Discussing the cost of pure transit isn't too useful. Heavy users smash both backhaul and transit if they go nuts, and this usage can be much harder to predict and manage than 1000+ users calmly watching youtube or emailing kids for 5GB/mo. It is much easier at large scale, but for a small, non-profit rural co-op I imagine the handful of big users they have could get very expensive.
In the pricing model, there needs to be an element of discouragement (suddenly heavy users with deep pockets can still degrade network quality for everyone else) as well as recouping the cost of required upgrades and improvements to support the traffic. It is ridiculous for a non-profit to massively overcharge on the scale you're suggesting for no reason.
I'm not sure what Net Neutrality from the summary has to do with any of this either - this is usage-based billing over a flat pipe, not charging/throttling based on traffic type or destination. I may be a bit biased as I'm from somewhere where quota plans are the norm.