I don't think the cost of the glass tubing sections anywhere is $25. As state said, you can buy just the tube sections for $5 each. Glass is heavy, and that's not a bad price, considering that someone has to buy the 60" tubing, then precisely measure and cut each section, THEN firepolish both ends, then kiln anneal them for a day.
Borosilicate (pyrex or the glass they use in Baccarat crystal) is pricey, but most important, it's heat resistant, which means it takes a long time to melt even at 1750 degrees F. You cannot speed flame polishing time by any mechanical means, it's time consuming. Nor will they precisely break at the seam, boro is STRONG glass, you need a diamond saw blade. $5 is not bad for the time it takes to do all that. Actually, I just looked at IBTank's site, and I think they are mechanically grinding the ends, not flame polishing. Are the ends frosted or crystal clear like the body? While grinding is MUCH faster, it still requires specialized equipment and time.
The $25 includes the end and bottom cap plus washers and O-rings ... which IBtanked sells caps for $17.50. There's your biggest cost for the finished tank.
Now if you can find those caps that will FIT a tube, that are cheaper and as good looking, I'd sure like to know where too

. I wouldn't use plastic caps ever. For one, it would take the point away from having a glass tank in the first place, since you can buy a new clearomizer for the cost of the glass part alone. Not to mention the risk of inhaling plastic if you use a juice that could dissolve it (and molten plastic can release industrial solvents that are potentially very dangerous).
I'm definitely looking for higher end metal caps (because I blow glass), so if you find any stainless caps for reasonable prices, I would like to know!! THAT would be where your savings would lie, finding metal caps (preferably stainless) at a size that will fit the tubes!