The "lowest cost" option is something called "ballistic steel". Note the "threat level" the first category of steel will protect against. UL 1-3 (NIJ IIIA) are handguns only. The middle category of steel is good for NIJ III and UL 4-6, which seems to be your typical military rifle fire. The last category is UL-7 will protect against high-power rifles.
You know, it's not like you go down to the "armor store" and buy some magic armor that will protect you. You have to determine an acceptable level of protection versus weight and cost. [more on ballistic specifications]. [How Armor Protects a Car] You must also factor in replacement cost, meaning if does the armor get "used up" if it is hit with a volley of bullets, or is it still useable.
So far, I haven't found who in the U.S. makes "ballistic steel". It's widely available from China, though. Memo to Rumsfeld: Buy certified level III ballistic steel at an affordable price Size: 8" x 10" Availability: Usually ships in 3-4 business days..
I'm suffering from a lack of interest over here towards #105 and #107. Maybe you two will find them interesting, especially the one about Clifton Steel.