New Harbor Freight Score!

Here's the old standby, the X-Acto #11 blade.
I've seen them in the hobby stores for as much as $5.00 for a five pack.
It seems they were always changed (before they were dull) when the brittle knife tip broke off.

Last year I got my "go-to" knife, the Olfa XA-1 at Home Depot for a little over $5.00. (There are cheaper break-off blade knives, I did want one that fit the hand and was sturdier.)
Next to it is a 10 pack of Olfa 9mm replacement blades for it. Also, for about $5.00.
There it 13 new sharp tips on each of the 10 blades.

When you do the math, each new break-off tip is just over $.04 each!
Since getting the Olfa knife I rarely use the X-Acto anymore.

If you've never used a break-off blade knife before - 
There is a little "play" in the knife blade. These are utility knives and don't "lock" as tight as the X-Acto style knife handles do. Like any new tool they might take some getting used to.
But for the cost savings and always having a sharp knife tip it's worth the compromise.

Harbor Freight sells a 10 pack of the 9mm blades for $.99!
Each new sharp tip is $.007 - less than a penny each!

The Harbor Freight probably aren't stainless steel like the Olfa brand.
Sure, the packaging isn't as pretty and there is a slight bit of corrosion on the blades - my balsa doesn't care. These blades cut just fine.