$36 for a proven, quality, and multifunction tach isn't really "outrageous" - tho I can see merit and certainly respect the do-it-yerselfer, even saving a few bucks but you really need a touch of perspective lol Have you priced replacing the entire tach unit on a new Honda motorcycle, or even just a taillight lens? heh, these TinyTachs are a great product for those that just want to stick a good, reliable gauge on their bikes w/o fuss or hassle.
Another way of lookin' at it is $15 bucks or so fer yer parts + say $10 per hr labor rate = about the same price as a TinyTach. The only difference being the justifiable pride in making something yourself.
They're for far more products than just planes lol - perhaps you've been shopping in all the wrong places
The reason they're so popular amongst ultralight aircraft owners is their absolute reliability - an airplane is no machine for failures and a tach is a critical instrument for flight.
Tiny-Tach: Gasoline Tachometers