Sewing machines are not and shouldn't ever be considered an "investment". The experts will tell you that an antique (100 years old) treadle machine in pristine condition is worth at best, $150. Harsh, considering what some of us have paid for our machines, but fact.
The thing is, none of our machines are considered to be "rare" ("rare' is considered to be fewer than 10 known to exist in the world) Some are less common than others, and as such carry more value to some, but go look at the serial number database on Ismacs and look at how many machines were manufactured...