HP EX920 M.2 256GB PCIe 3.0 x4 NVMe $46.99 at Newegg

[trs96]

https://www.newegg.com/Product/Product.aspx?item=N82E16820326777

If you've got a x4 M.2 slot onboard you can get 3GB/s read speeds and boot times under 10 seconds, maybe faster.
If that kind of speed is important then 47 USD is a small price to pay to rev up your boot drive. This is a great choice if you edit 4K video in FCP X or Premiere Pro on your CustoMac. You could use it either as a scratch drive or your boot drive.

 

[trs96]

5-24-19 Update: Price has gone back up to $46.99

Price has dropped by 8 dollars since the OP. If you've got an M.2 NVMe slot onboard this a no-brainer.
Many 256 GB Sata based SSDs go for about 35 dollars.

 

[trs96]

1 TB size is also on sale for $133 if you need to go bigger.