Couple Ideas come to mind. First, if you want light and portable, you could try closed cell foam. It is commonly used as archery targets for field points and broadheads. Knives will probably chew it up faster than arrows, but it would fit the bill. Cost can be from @$10-$40 bucks retail.
Next, for less money than foam- reinforced pallets. These are often free or cheap to come by. You could then attach 2x4s accross one side to provide a more solid target face. A little heavier, but still portable and longer lasting than foam. Cost can be from $0-$15 to make.
Lastly, and my personal favorite, tree trunks or tree rounds. This is the cheapest way to go, but depending on where you live, possibly a little more labor intensive. I cruise craigslist for free firewood/scrap wood. Here in the country, someone is always chopping down or chopping up a tree. They are often cut into 15-20inch rounds, which make excellent targets. I will load my pickup with as much as I can and stack it near my outdoor firepit/knife range. I don't care if the wood is green or dry for knife/hawk throwing. Once it is chewed up pretty good, I burn it in the firepit. I have gotten creative and made some leg stands to set the rounds on to bring up the height to a more desirable level, but it is not necessary. I will sometimes spray the pile/target with bug spray or just sprinkle bug dust arount the wood pile. That keeps the critters at bay. Occasionally I'll impale a malingering spider or earwig that strays onto my target surface...entirely by accident.
Have fun and good luck!
