This does depend a lot on geographical location. You're not going to get free racks in a major carrier exchange point in Manhattan, or downtown Seattle, or close to the urban core of San Francisco. There will definitely be a significant quantity discount, as you increase in numbers, and a lot of your cost will be power and not racks.

In somewhere that is very close (OSI layer 1) topologically to a major traffic interexchange point, you will definitely be paying somehow for the monthly cost of the square footage occupied. For example a colo in 60 Hudson or 25 Broadway in NYC, or in one Wilshire in LA.

Large-scale colocation pricing that is based on power only will be found in locations that are not also major traffic inter exchange points. For example quincy WA or the many data centers in suburban new jersey.

I think your point is right for 111 8th or Equinix/SV1/Great Oaks.

But for other sites with high value peering, including EQIX in Ashburn, Coresite, your limiting rate is likely to be the power, not the space or power. I.e. they'll "give" you 1 cabinet for every 15kw you buy.

So my assertion assumes you're doing large number of dense cabinets.

If one is doing a large number of dense cabinets, they almost certainly should not do it at a high value peering point, and should backhaul it. You should be able to get diverse metro dark fiber for <$5k/mo if not substantially less. Put a single cabinet (or pair for redundancy) of $2k/mo cabinets in Equinix or Fisher and off you go.