[quote name='Maskim']Where would you all suggest looking for building plates at? We've got one that I took out of a lego board game I found on clearance for a couple dollars last year, and all my kids fight over it every time they bust out the legos.[/QUOTE]
Those damn plates are easily the biggest ripoff Lego wise nowadays, at least IMO. I'm just happy I have a couple hundred or so, maybe more, different plates and bases from my childhood (tons of grey, blue, and green plus all kinds of city streets and lots of the lunar crater ones). But I'd say if you can find them used, say on craigslist, ebay, or at yard sales et al would be the best bet for cheapest prices.
Unfortunately I didn't get many of the specialized base plates that had different height features that came out generally in the 90's and some in the 2000's. When I look at prices for the plate packs and single large ones today I just shake my head, simply ridiculous. Cost of plastic be damned. Might even be better to buy one of those Lego tables that include one or more of them (glued on?) that they sell at Toys R Us.