I've just noticed the new BX500 drives are now on eBay. Slightly cheaper than the MX500's yet similar performance possibly faster..
Comparison of the BX300 to the MX500(The BX500 cannot be compared with the BX300 or the MX500. The BX500 appears to be a QLC DRAMless SSD. The BX300 is a 3D-MLC DRAM SSD. The MX500 is a 3D-TLC DRAM SSD.)
http://ssd.userbenchmark.com/Compare/Crucial-MX500-250GB-vs-…
and a news report with some good looking numbers on performance..
https://www.guru3d.com/news-story/crucial-silently-launches-…
Look like the BX series is the budget range from Crucial.