Warping is the one issue I am concerned with when basing my terrain, and like yourself I am looking at doing things based around 150mm by 150mm or 300mm by 300mm squares for my modular city.
I am looking at acrylic or perspex myself, should be stronger than wood, foam or card and less likely to warp.
Near me there is this place that cuts acrylic and perspex sheet to size, which I am going to try out. Prices work out in the region of:
3mm thick black acrylic, 150mm by 150mm = £0.88
5mm thick black acrylic, 150mm by 150mm = £1.46
3mm thick black acrylic, 300mm by 300mm = £3.51
5mm thick black acrylic, 300mm by 300mm = £5.85
Clear acrylic is even cheaper:
3mm thick clear acrylic, 150mm by 150mm = £0.51
5mm thick clear acrylic, 150mm by 150mm = £0.77
3mm thick clear acrylic, 300mm by 300mm = £2.05
5mm thick clear acrylic, 300mm by 300mm = £3.06
Comparing the above to Litko (10 of 3mm black acrylic, 150mm by 150mm $143/£93!!!) it seems massively cheaper
Obviously this is a UK based shop, but I would have thought there would be someone similar in the US who is doing the same thing? A quick google throws up these:
http://www.vipplastics.com/product_info.php?cPath=57&products_id=1008http://www.delviesplastics.com/mm5/merchant.mvc?Screen=CTGY&Store_Code=DPI&Category_Code=125_Cut_Acrylic_Sheet_PageMight be worth a look.
Hope that helps.
Matt