@ShawnR -- Your level of experience and expected accuracy will determine what will make sense in a surface plate. For a beginner or intermediate to do workmanlike accuracy to say 1/10 of a thousandth of an inch, both my Busy Bee surface plates at 39$ and 49$ will do the job, it isn't worth the milling, grinding and lapping to make a solid steel plate. I also acquired a Mitutoyo AA surface blate that was bigger for more accurate work.
If you want to go bigger and better the smaller name brand granite plates are reasonably priced, and granite doesn't rust.
Frankly when the time comes I see that hunk of steel being a great table for a belt sander or filing machine. You can make T slots in it and have enough meat left over for rigidity.
-- in fact I'll be making a new shaper table next year and will need a piece of mild steel in the 1 1/2 in thickness range... hmmmm?