Expensive ones are usually 540MB/s write speeds while cheaper ones go up to around 250MB/s write speeds only.
Read speeds however, are the same.
False. Baseless assumption.
As an example of the newer cheapest drive you can get. This is something I wouldn't expect from WD or from any storage manufacturer.
*www.hardwarebbq.com/wp-content/uploads/2015/12/Kingston-UV300-AS-SSD-Write-MBs.png*www.hardwarebbq.com/wp-content/uploads/2015/12/Kingston-UV300-AS-SSD-Read-MBs.png
And the only reason why newer premium variants are expensive is because of the channel and the amount of lanes it provides along with the storage capacity, read/write speed and the manufacturing process.