Yes, negative is rare, but I wouldn't say that it's overwhelmingly expensive.

The median range is 15p-20p (60% of the time in December) and the UK "price cap" is about 26.35p.

With a tariff like that, shifting usage outside of 4pm-7pm can lead to massive savings. With our usage from the Octopus API, I can see from OctopusCompare that in the past month my effective average unit cost would be 19.24p/kWh, and we don't do any specific load shifting.