Let me say this bluntly and get it out of the way: if you're concerned with finances, this is not the hobby for you. The cost will VERY quickly rise into the hundreds of dollars, along with hundreds of hours, so unless you have some extra cash that you don't need, you will most likely end up junking the project halfway through.
If you aren't concerned with finances, though, then go to Wal-Mart in the office supplies section, and look for a ream of 150 pages of 110lb cardstock. It's around $6-$7, and will last you a decent amount of time. If you have a Staples or OfficeMax near you, even better, you can get 250 sheets for about $10-$12.
Don't try to take shortcuts with armor making, you will lose many hours of work because of it. Buy the cardstock, glue, etc., build a piece, then continue on as outlined in the tutorials. If you're apprehensive to even start your project, you will not finish. The pepakura process is the tip of the iceberg and is a tiny fraction of the total cost of a full suit or helmet.
Not trying to sound rude, just laying it out straight so you can decide without all of the frilly, cute, positive connotations. If you need any help, feel free to PM me with questions