£1 million in £20 notes stacked is around 5.65 metres high.
£1 billion in £20 notes is 5650 metres high or about 2/3rds of the height of Everest, at around 3.5 miles.
The bridge cost around $60 million to build in the 1970s. Even with inflation that would only be about $450 million today. So $1.9 billion is insane.
450 million in inflation isn't equal for everything.
Remember "Inflation" as a headline number is an average of a lot of things.
for example the cost or certain raw materials and finished materials may go up far faster than any "average" inflation number, whilst other things may go up at below the inflation rate. It's one of the reasons that if you plan a build for say a house and you allow a 10% margin of error for materials costs, even over just 6-12 months you can go way above that if say the cost of fuel goes up or the cost of say a steel beam goes up (and IIRC steel has several times gone up in 6 months at multiples of what the inflation rate was*). IIRC it is currently cheaper to use actual proper wood than the likes of plywood for some projects at the moment because the cost of that has risen so much faster than the cost of some woods, and some particular cuts of wood went up massively faster than inflation because of things like the mills and shipping being affected energy prices.
Also they're likely going to aim to build it much stronger (extra cost) and with better resilience (extra cost), and likely to a very different design (extra cost) and do a lot of computer modelling to see how the design reacts under various conditions (extra cost, but cheaper than as has happened in the past, you build a bridge and find it's unsafe in certain weather conditions).
Even things like fairly simple seeming design changes can have a massive effect on the cost
*One of the main reasons every major multi year project seems to go "over budget" is because if they say have 3 months in planning disputes/court disputes over the route you can see the cost of materials having gone up before you've even started getting them on site, and that means you're now looking at inflation affecting everything more than allowed for, let alone the odd war disrupting the energy costs and shipping routes.