I was in Woodcraft this weekend and they had a sale on their wood river surface plate for 27 dollars. I picked it up on a whim.
It is 9x12x2 and shows an accuracy of 0.001 mm/m or 0.0001". the inspection report inside the box states The ACCURACY (yes, all caps) to be 2.4 um, 00 Level.
I am thinking it will be sufficient for my little Sherline and unimats.
Funny thing, the sales guy was pushing me to get the larger one for 80 bucks so I could adhere 2 pieces of sandpaper to it. He was surprised I wanted to use it as a reference surface for more accurate measurement.
I figure for 27 bucks, it is hard to go wrong.