These products had nicely controlled market prices until September 2017 when they all hit the roof and increased dramatically. Before this, prices were generally below £2.00. After this, the prices were over £30.00.
Since then, market competition has driven prices down to below £20.00, but nowhere near as low as they were initially.
The long-term price history came from BPPI.co.uk and the current data from July came from wavedata.net