The UK and the US don't use the same octane rating system. What you're buying in the UK as 95 octane fuel would be 91 octane in the states, which on the eastern seaboard is sold as mid-grade. "Premium" grade fuel in the east where I live is 93 octane, which would be equivalent to your grade of 98 octane. Europe uses the RON (Research Octane Number) system, while the US uses the PON system (the Pump Octane Number, which is the average value of the Research Octane Number and the Motor Octane Number).
A gallon of premium (US 93 Octane) is currently about $3.11 a gallon where I live. Regular gas (87 Octane) is about $2.49 a gallon. The price varies quite a bit from state to state (and even county to county) based on transportation distances, taxes, and other factors.
There's a myth that high octane fuel is somehow "better" than lower octane fuel; probably because here in the US they call it "premium". Guys will talk about how they "treated" themselves to a tank of premium. I believed the myth myself at one time in my younger days. Putting high octane gas in an engine that doesn't require it is just a waste of money.