The average American spends $5,000 a year on the total cost car ownership, completely separate from public spending on roads and infrastructure, so I think that signaling cost is reasonable enough, especially since signaling will last far longer than the typical lifespan of a personal vehicle.

£5200 for your annual ridership but then divide that by the lifespan of the signaling. Let’s say it lasts 30 years, so that’s €173, (but of course, at the end of the 30 years £173 is worth a lot less).

How much an American spends on a car every year seems not that relevant to how much a British tube line signal upgrade costs. Why not compare it to house prices in London, and it'll seem even cheaper?

I compare it to automobile infrastructure because public transit infrastructure projects sems to be the only time anyone complains about high infrastructure costs.

The only reason I compare it to America is because I don't actually know the figures for the UK, but I expect that car owners would spend more there due to higher cost of petrol, higher vehicle taxes, etc.

Cars are unreasonably useful, though. If you're going anywhere to work / school / moving house / going on holiday / dropping kids at a friend's house, they work for a million uses.

That's why I don't think they compare well: many people who use public transport, even the Tube or similar, still need a car, at least sometimes. So it's not an alternative, and it's not equivalent value.

So probably better to pitch it not as £10 per rider (~200m annual riders) but £0.33/trip levy for the next 30 years.

What do you mean by missed opportunity? To not run it there more frequently? Can't they still do that? The timetable has seen some updates already.