Snazzy Labs built a startlingly powerful Mac with only $70—editing the video above on it to prove it!
A big part of it is being armed with knowledge and some technical chops: the donor PC (from institutional surplus) was a real beast in its day, they repaired a broken SSD (!), and found a miraculously appropriate bus-powered video card on Craiglist. All of which means you're not likely to replicate the "Crapintosh" in a $100 visit to Goodwill.
All the same, though, I still think this shows something interesting. Not that MacBooks are underpowered, but rather that traditional desktop PCs, unfashionable as they are, are a cheap way to serious computing power if you're happy with an enormous noisy box under your desk. You might know this, but processors and video cards alike have long been marketed to obscure the performance differences between mobile and desktop parts, and they are often substantial! So educate your thrifty friends.