On an episode of "The People's Court" I recently watched, a viewer posed the question of the judges, "What is the best $20 you ever spent?" I said something about John had better relate something to Marilyn, and my SO says, "Can you remember the best thing you spent $20 on?" I thought about it for a few seconds... when you get to a certain age, you can drop a $20 on a lot of things, and sometimes the best expenditures are larger amounts or the value has a long temporal separation from the initial investment. I landed on this: the TI-34 calculator included with TI's Computer Math Kit.
I bought this in 1988 when I was in 9th grade. It cost me $19.99 at Montgomery Ward and was the first major purchase with long-term implications I ever made. Using the book which came in the kit, I learned basic statistics, geo-coordinate systems and calculations, and more. The calculator can work in hexadecimal, including conversions between hex and decimal, octal, and binary, logical operations like OR, AND, XOR, all which I used gratuitously in my programming days while learning TMS-9900 and 6502 assembler. It also works in fractions, can do complex equations, trigonometric functions and systems. I used this calculator all through high school and my years before college. It is obviously well-used and beat up, the cover broke many, many years ago, and the book is somewhere around here, but it still works.
I recently picked up a couple of pristine devices and a full kit to rebuild it, but there is something about the patina of original use, the charm of the discolored keys, scratched up solar panel cover, and rubbed off lettering.
This listing on eBay, coincidentally for $19.99, shows the full kit as it originally appeared in 1988 when I bought mine.