Not as cheap as a piece of aluminum plate. If you are doing a lot of experimenting it makes sense. If you are designing a projector, it is a waste of money to pay for tapped holes that you won't use and will allow dust to go through.
Dnar ----
You can buy an aluminum plate to your specs cheaper. If you have a drill press and measuring equipment like calipers, you can do it yourself. I used layout fluid for scribed lines. Kerosene makes a good tapping fluid too!