If you go to the pc part picker website select ram then price low to high filter to DDR4 and 16 gb and go for the cheapest you find. just have to cough up there is no outstanding value at current prices.
I got Hyper X 8GB DDR4 2400 for around £70 the prices scale almost perfectly to double the cost of 8gb more often than not.
unfortuante, but just how it is for the time being.