Small projects are also a problem. And I think shiny fancy etc is part of the problem. For example, TfL were quoting £700m just to upgrade the pedestrian access to Holborn station. There is an awful lot of stuff like that which needs doing across the London Underground network, and it needs doing within the next few decades, not centuries. I think they now realise if such is the cost of dealing with just one station, then they just haven't got anywhere near enough money in the period of time that is required, and large parts of the network are just going to fall apart through lack of money to fund renewals in anything like the timescale required. There just is nowhere near the amount of money to put so much of it on just one station, relatively important station (nearest to my office) though it is.
A couple of years ago I went to a conference on railway station developments, both in the sense of building new stations, and also using railway stations to encourage economic development. The Dutch head of railway stations and some colleagues were there and made a presentation. Alongside was a presentation on the new railway station at Meridian Water, a new £46m commuter/local railway station serving a large development area in the Lee Valley in North London, on a 2-track railway that required no realignment. The railway station at Southend Airport, a £30m station on a 2-track railway requiring no realignment, came up. It was clear that the chins of the Dutch contingent had clanged to the ground, long way though it is for a Dutch chin, at the costs of these projects. They thought it was a long way beyond stupid.
I believe Chiltern Railway succeeded in building the actual station at Warwick Parkway, a new station on a 2-track railway, as part of Project Evergreen for less than £1m. The car park and access was separately costed, so that's makes the comparison somewhat misleading, as that would be included in the cost of the other projects. It did subsequently get a bit of smartening up. But it makes you wonder what the difference is between a £1m station and a £46m station, on a plain, straight 2-track railway.
The Dutch mentioned the Luchtsingel, a 400m footbridge, with multiple entrances, creating pedestrian connections across several thoroughfares including a 4-track railway and an 8-lane road, assisting with the revitalisation of some areas near Rotterdam Centraal station which had become severed. I came away with the impression it had been built for €400,000. That contrasted very badly with UK costs, as you'd struggle to build a basic footbridge over a 2-track railway line in a small town for that in Britain. Though I now have difficulty getting a source for the cost, and I suspect that was the cost for just one of 6 phases of construction, or perhaps just the design phase, and it must have cost a lot more. Though it would be nice to find the actual cost, and then we could try to compare it with spending £50m on the Garden Bridge before even putting a spade in the ground... I'm afraid google didn't help, but maybe someone who knows more Dutch than me could find something.