Foacore is very expensive in Austria.
Look at :
50x70cm 5 Euro!!

That means, only the foamcore would be much more expensive than the whole set: ~30$ for a design the size of a masterboard!!

@Morgenstern: I mount my fixed-size rectangular tiles on 3mm MDF board from my local Baumarkt (Hellweg, OBI, whichever is available). They usually offer cutting the stuff to size (+/-1mm) at no additional cost, provided it's rectangulars at least 10 cm by 10 cm. Unfortunately they don't do inches, but if you do the maths based on 1 inch=2.541 cm, everything is fine (a 6 inch square is 15.3 cm square).

MDF is quite durable, not too heavy and glue-friendly. Go for the uncolored stuff with two plain surfaces, it's usually les prone to warping than foamcore, and it's quite a bargain. Not long ago I had to make about 100 six inch square tiles, and I paid roughly 10 Euros at Hellweg. Plus, if you're ever doing 2D tiles, you can get 2 out of one tile (glueing the second print to the flipside, which further prevents warping). The greater weight ofthe MDF tiles also means they won't move around that much if someone bumps against the kitchen table.

I used cardboard on my first board and it warped like crazy

I was light on the glue, and weighted it, but it still warped. I have been looking for an alternative since I've got to Spain. What is that stuff in the link? My German is bad... ok, it's non-existent and my brain is mush from full time Spanish study

The stuff in the link is just foamcore.
(Leichtschaumplatten = "lightweight foam sheets" = foamcore)

As I mentioned above, I recommend 3mm MDF (medium density fibre, some kind of very smooth wood compound that usually doesn't warp) for rectangular tiles sized 10cm/4'' square and up. Heavier than cardstock or foamcore, but durable and forgiving.

Haven't seen them anywhere so far. Next problem, they probably wouldn't be 12'' by 12'' here, because you basically only get metric measures, inch-sized stuff is almost impossible to get (try buying an inch-based measuring tape in a German store and you know what I mean - luckily there's RPG traders around that have tapped into that business a couple of years ago...).

I'll go and check some Euro (Dollar) stores next week, maybe I'll find something useful. Anyway, I get my foamcore sheets (50x70 cm) at about 2.50 Euros, that's okay. But yes, there are some shops around that sell it for up to 4.50 Euros.