I mowed lawns and did other yard work, but that was back in the early `90s when gas for the mower cost a lot less! Then again, the few knives I did own were a far cry from Benchmade or any of the brands I like now. More like United Cutlery, if a name brand at all. I still have most of my knives from back then, and none could have cost more than $20.
Something to keep in mind is that, unfortunately, your age may work against you. A lot of jobs will simply not be attainable to you at 14, whether because of legal reasons, the nature of the work or simple factors like not being able to drive. By the time I was old enough to be earning steady income around 15-16, my interest in knives had been put on the back burner so that I could save up for my first car, and I didn't buy a knife again until I was well into my 30s. My first $100+ knife purchase was within the past year or so, and while I'm not a teenager anymore, I have something that a lot of other guys can relate to: a wife.