Well if you're planning on just using a 1/4" - 1/2" compressed wood board to cut the shape of the mat, you're looking at around $5. Hinges can run you anywhere from $2/piece - $10/piece. Depends on what style you want. But they're all pretty basicially the same.

If you rather go with some 2x4' support beams, just get about 2 beams. Costs you no more $5. Don't need to get the fancy stuff.Carpeting is totally up to you. You could always use the existing mat and just place it right on top.