Originally posted by ling.speed:

The "only weapon" hehe... Almost all weapons have that feature. Starting from Flint Axe.

Well, read a few more words in and you'll see the statement was "only weapon I've made...."

Typically, the first upgrade IS actually less. Second upgrade usually uses about as much of the "main" material as crafting it, and 3rd upgrade typically costs 150%. It's not a universal formula, but it's more or less what to expect. Notable exceptions are the regular club and stagbreaker.