After some thought, I've realized that there are two aspects to the question I originally posed.
On one hand you have the comparison of value and price between firearms and knives. This one is entirely subjective since the value of something is not determined by any inherent property of it, nor by the amount of labor required to produce it, but instead value is determined by the importance an acting individual places on something for the achievement of their desired ends (Wikipedia). I think Scurvy and A Justice hit the nail on the head as far as comparing knife/gun values.
On the other you have the perceived vs intended use of firearms and knives, which is what I was really thinking about when I made this post. In that case it has more to do with people perceiving the knives I carry as intended primarily for self defense.
I won't say that I don't carry knives for self defense purposes. But it's more of an afterthought. The reality is, that if I found myself needing to use one of my knives for self defense it probably wouldn't turn out well. Not that I don't have confidence in myself, it's more the fact that criminals these days tend to carry a weapon anyway so it's either me with a knife vs a badguy with a knife (in which case, both of us are going to get cut) or me with a knife vs a badguy with a gun (and you know how the old saying goes). I guess what I'm trying to say is that, to the folks who tell me I could buy a gun with the money I spent on a knife, it's really apples and oranges.
A great example is from when I used to play tournament paintball. After playing with some brand new guys, they asked me how much my setup cost. I told them. My marker, tank and hopper cost around $1200. Their response was... "You could buy a real gun with that much money."
Well, it's clearly another case of apples and oranges in that case, their perception of why I played paintball was completely inaccurate. Much like how some people's perception of why I carry a knife is inaccurate.