Ive been building an Iron Man costume out of foam for about 6 months now, and I have to say it really depends on the thickness your working with and the intended result. Most camping mats are about 1/4 to 1/2 inch thickness but I would advise the EVA foam if you can find it. Its much easier to work with for a smooth result, then all you need is a heat gun to bend and shape it and alot of patience. A 1/4 inch mat in 90x60 is available is stores like Sears for $24 a roll and if that gives you an idea that will build about half of an Iron Man suit so its great for budget builds. You can even use it to build some of you prop weapons and other items.
I will suggest the EVA more than the camping mats because one side is textured and one side is smooth, no real need to apply 3 layers of sealant, 2 coats of resin, a coat of fiberglass and a butt-load of bondo just for a smooth result, theres about $200 down the drain right there. just blast it with a heat gun to give it a sheen and to firm it up, then apply a few coats of sealant (equal parts tacky glue and water) then paint and clearcoat it. saves you time and money in the end. here is a picture of my Iron Man armor to give you an idea as well. If you have any question on foam working shoot me a message and I'll gladly assist as much as I can, best of luck with your cosplay.