Originally Posted by 7AC
This is late, but I'm surprised no one mentioned the razor knife available at dollar tree. For $1 you get the knife which has 7 break-off blades and a pack of two more break-off strips for a total of 21 blades. Buy two and you can cut foam for a long time.
Been there, done that ( purchasing their cheapo knives I mean) the only problem is that they use a non standard size blade, so when you break them , or run out of snap off sections, you will have to throw it out since there are no replacement blades that will fit. But hey, what's there to loose with a 1$ knife, other then some skin when the low quality blades break when it's least expected.