I think the dinner table mats would not work because they would be too thin. But if you could pull it off and it looked right, it would rock because you would be really flexible. Goodluck on your quest, and don't be afraid to ask!
$15 - for cutting blade, primer, sand paper, and the Rub-n-buff (these will need for no matter what you use as a base for the armor. you may have to add or sub out item depending what you armor is made out of.)
Each 2' x 2' sheet of expanded PVC was about $4.38 I bought 8 sheets (about $35). You only need 4.
Unless you have all the detail supplies, they will cost about $15 leaving you $5 to find base armor material. The devil is always in the details and so is a alot of the cost if you are just starting out.