I personally spoke to Holland BPW multiple times over many months in late 2013 about bringing gigabit fiber to Holland using the BPW fiber. At that time, they were asking roughly $500 per month per location, just to connect a single customer to the network. And they'd only let us deliver a max of 50mb or 100mb (they wouldn't let us offer gigabit in any fashion, full stop).
That's why "getting a 10mb connection from a third party ISP costs $600/month" -- BPW has historically been the biggest roadblock. I can't speak for the other ISP's, but I strongly suspect BPW's fees alone might account for 50 - 90% of that monthly cost, if other ISPs are paying the BPW line fees we were quoted.
Holland BPW doesn't actually need to become an ISP. If BPW just lowered the line fees to something reasonable, multiple providers would rush to provide fiber internet to Holland, and there wouldn't be any legal challenges or problems at all. Utah has a good example of this working in practice :
https://www.utopianet.org/pricelist/
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Holland BPW has been the largest roadblock preventing internet from happening in Holland. Have you spoken to them about this? Have they changed their stance in some way?