As part of the Total Route Modernisation of the Chiltern route (which wasn't 100% total) just one platform at Princes Risborough remained in use (the down platform) and the island platform was closed and the footbridge to it demolished (1987 I think). After privatisation when the line between there and Bicester was redoubled there was a delay in bringing a second platform into use, and it was reported that a new footbridge, with lifts which the previous one didn't have, would cost £100,000. I think you almost need an extra 0 on that amount today.
In the redoubling the existing single platform at Haddenham and Thame Parkway was rebuilt by cutting into the embankment on the SW side (it's now the down platform). My recollection is that the job was done in a closure of 17 days, but Adrian Shooter was reported saying that Railtrack originally had wanted to take months over it, which with compensation and additional alternative bus arrangements would have made it much more expensive. Doing the job quickly meant that the platform is built of precast trestles with slabs over the top, similar to the platform extensions on the West of England line to accommodate class 159s. Many newer platforms are now built much more solidly, but those cheaper versions work perfectly well.
Later, they built the new second platform at Haddenham and Thame (the up platform). It doesn't have a lift but it does have an extensive ramp structure which feels as if it goes on for miles.
Where I live at Overton, 70% of the car park - for about 120 cars - is on an area with a crumbling tarmac surface with potholes, much of which is privately owned. About ten years ago Network Rail quoted £700,000 to provide a new surface with proper drainage and lighting. The subsoil is chalk and the site is on a slope, so drainage is easy and soakaways straightforward to construct. So nothing has happened.
I think the main reason for the high costs of modern stations is the desire to build them to a very high standard, some of which is necessary to provide access for passengers with disabilities, while other factors are a kind of corporate grandeur, a category which includes the massive steelwork on the GWML electrification. .