The hardware/bandwidth cost numbers floating around are low. But they don't allow for time and management overhead. My take is 4000pm will be where we come in, with paid servers looking after 2000ish users because management overhead is essentially a fixed cost from when you have to commit to full time, to when you hit the number of users a "one of you" can support. That number MIGHT be in the tens of thousandss, and if so, the cost per user will fall. Long term when Relays are Utilities, probably 1000-2000sats per month. Remember Relays are the only people in the system that everyone with a copyright grievence or legal phaff can find and sue and so dealing with that crap will be unavoidable, and have to paid for.

As every garage mechanic ever always says, "it's not the parts, it's the labour" :-)