$500 is pretty reasonable, especially if it's in good shape, but I agree, you're better off putting that money towards something better. Both the quality and the range of bake times it can produce are question marks, imo.
The plate that I'm referring to is A36 hot rolled steel plate found here:http://www.onlinemetalstore.com/items/A36_Hot_Rolled_Steel_Plate.cfm
This is readily available at a local metal supplier. Just look up steel in the yellow pages.
The power of steel plate is that it's conductivity allows for much faster heat transfer, which, in turn, allows it produce much faster bakes at typical home oven temps. Basically, 1/2" steel plate preheated to 550 can mimic the effect, hearth wise, of a 1.5" thick cordierite deck in a commercial pizza oven preheated to 650. 3/4" steel plate at 550-600, because of the extra thermal mass, can mimic the 2" firebrick floor of a Neapolitan woodburning oven at 850.
The only real downside to steel plate is that it can be heavy. Most oven shelves have no problem supporting 1/2" steel plate, but 3/4" can be too much to handle. In order to work with 3/4" you basically have to remove the shelf from the equation and fashion your own support of an angle steel, running from shelf lip to shelf lip.
Both approaches are less than $100 scenarios, so, rather than spending either $500 or $5000 on any type of oven, wood fired or otherwise, it's worth the expenditure, especially for NY, as that's developing a fairly loyal following.