Pilots will undoubtedly let their in-flight decisions be influenced by the cost of diverting
Sad, but probably true in some cases - although hopefully not all. Having only had to make a diversion once (which is still once more than I'd have liked), all I can say is that the cost was the last thing which entered my mind. In fact, I'd have been quite happy to pay the landing fee, even though it wasn't required because the field I diverted to was one of those 153. After all, I never made it to my destination, so I saved the cost of the landing fee there...
There are a few airfields where landing is stupidly expensive, and which aren't part of this scheme. Places like Heathrow, Gatwick etc. All of these places have radar. I have no doubt they'd let you in if it really was the only option, but I also suspect that they'd offer as much help as possible in getting you somewhere else safely if the option is available.
In fact, only a few months ago the whole of south-east England was covered in fog, except for the Heathrow area (which is a little unusual to say the least!) I was in the operations room at White Waltham when we got a phone call from London Radar to tell us they were vectoring an aircraft through their zone to us, he'd be approaching along the railway line and they'd hand him over to us with about a mile to run. Shortly after that a very relieved Cessna 172 pilot and his family landed safely, having been unable to make it into Elstree.
FFF
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