I don't really mend clothes. I've done some minor alterations for my teenage daughter (shortened a skirt, fixed a zipper). But my time is usually worth more than the cost of replacement (especially jeans, if they're on sale). My rude and ungrateful stepson has shown up ON Halloween asking for alterations to whatever costume he bought at the party store. Last time, I used safety pins.
My daughter's school has sucked me in, however. I am their costume designer/creator. I've made about 40 medieval outfits, 5 Ancient Greek, and a dozen or so 1930's-era. Those required altering a whole lot of thrift store purchases.
On the up side, they are finally paying me a small stipend, and covering the cost of materials.