Just one costs upwards of 30 dollars, US. They're supposedly difficult to breed in captivity, and take 6 months to mature. Adorable, but that is an expensive isobuddy.
But it's not an off comparison, you can get up to 20-30 common variety isopods for $10-15 US. When you're looking at setting up a stable breeding population for a fairly self-sufficient terrarium, that's going to get expensive with Rubber Duckies.