I use mdf on both mpcnc and lowrider. If you use half inch, then you can resurface a few times before replacing.
Depends on how bad you do your CAMMING and how deep in the spoilboard you chew when cutting 
I did have a few spots where I accidently dropped my surfacing bit almost half way through the spoil board. Those never surfaced outā¦
My current board is 20mm. Leaves plenty of depth to wreck up. But Iām not always careful to use screws that are short enough, so I think that the bottom board has a few holes. No big deal, they are meant to be used!
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This is the big takeaway here. Itās a 'spoilāboard for a reason. Itās going to get destroyed and is there to keep the main build platform from being wrecked.
Personally, I use MDF as well. Itās cheap and easy to drive screws into.