Gravitricity

A subway tunnel might have a diameter of about 6 meters, so cross section = 3 * 3 * pi = 28 square meters. Digging subway tunnel through rock costs about $100M per kilometer. On the one hand, these holes would be vertical, which is harder than horizontal; on the other hand, they wouldn't need ventilation and train tracks and stuff. Let's handwave and say it's $100M for a 1 km deep hole.

Now, you can't fill the whole tunnel perfectly or the air can't escape, so our total volume of mass will be about 25 m^2 * 1000 m = 25,000 cubic meters. If the weights are made from lead, that's a total mass of ~280,000 tons or 2.8 * 10^8 kg, at a mean depth of 500 meters, so our total potential energy is 2.8 * 10^8 * 500 * 9.8 = 1.4 TJ, or 1.3 TJ net assuming you get the efficiency they claim. 1 kWh is 3.6 MJ, so you can store ~400,000 kWh at $100M capital cost (ignoring for the moment the cost of weights, generators, etc.), which is $250 per kWh installed capacity.

That's pretty good... but you also have to pay for weights and a bunch of other stuff. Bulk lead costs about $2,000 per ton on the current market, so that's $560M for the weights, which puts you back in the $2,000 per kWh range which doesn't beat lithium batteries. So you have to use iron or some cheaper material... but then you don't have as much storage capacity because the density is lower, and even with iron you're paying $400 per metric ton or $80M for all your weights. So this isn't obviously impossible like Solar Roadways, but even in the best case it won't make storage dramatically cheaper.

On their webpage they say " The biggest single cost is the hole, but it is expected that firstly this will have a very long life and secondly, as the technology rolls out, the costs of drilling will reduce significantly. So the economics will improve in time."

Drilling will not get cheaper without a serious reform of the law, and that's unlikely to happen. There are two primary kinds of drillers; water and energy. Folks involved in water have VERY protective rules in all states. The reasons are 1. good lobbying and 2. if you screw up you destroy the water supply.

Energy drilling is no cheaper. Drilling rigs are huge machines that aren't moved easily and cost at least $50k a day on land and $500k a day for seagoing. That's before you pay another $20k per day in staff and god knows how much for fuel for everything; a rig will produce at least 5MW of power.

Could they custom build a rig just for drilling for their idea and would that work? Sure. But they'd probably spend several million and then you've got to have it running 24/7 for it to pay off.

The regulatory hurdle is going to be non-trivial as well. They're going to case it which helps. But the casing would probably need to be cemented in place which is a non-trivial cost as well. And convincing lawmakers that this doesn't quack like some other kind of well is going to be no easy feat.

Edit: You assumed that they can both fill most of the tunnel up with lead and move that up or down 500 meters. The volume that they can fill with that is thus only half of what you say.

I would have thought that a better comparison would be oil wells, which cost about $500 per ft of depth, or $1.5M per km.

our total volume of mass will be about 25 m^2 1000 m = 25,000 cubic meters. If the weights are made from lead, that's a total mass of ~280,000 tons*

You seem to be assuming that the entire depth of the shaft is filled by weights. My impression was that the weight was much smaller than the shaft it fell down.