It really does depend on the size of the tube and how much weight in concrete it is designed to support. For my last sub project I only had to get a 4' long x 1' diameter for a car sub. It cost me ~$7. But when I built the one for my home I had to buy a 10' long x 2' diameter and it cost me ~$80.
The tube for the car sub I just bought @ Home Depot and the one for my home I bought @ a lumber yard.
8" and 12" are the most common tubes and some home hardware stores carry these dirt cheap compared to the other sizes (in my area Wichita, KS one store sells a 12' length of either at less than $15) but the other sizes (diameter) are usually sold in minimum lengths of 12' at $2.50/foot (with some stores charging more for the extra large diameter of 32"+). My advice is contact shops in your area and maybe get a friend to go in as well (they might want a sub as well) since these usually are high $ to ship. I know one store offers one free cut but the quality of the cut for a sub purpose (flat and level) might not be adequate so buyer beware.
If you want to drive down to San Jose there are several options down here. I just got my 18" diameter sonotube 45" long for $17 cut straight and everything. I got it from Orco. Check your phone book for concrete forms/accessories. The average was $5/ft for 18" here. Orco had it for just under that.