yep, pet mountain has the best price on purigen. I buy the 1 liter bottle, lasts a while. Problem is you need to double bag the purigen in regular filter bags because regular filter bags are too porous and let some of the purigen out. Or if you can deal with a little purigen in your tank, a filter bag is fine
THe 100ml of purigen comes in its own filter bag and is rated for 100 gallon
As much as I like pet mountain for mopani wood and a few other items, kensfish has a little better price on the purigen 100ml (among other things.) I picked up a 500 ml jar and 2 seachem media bags on e-bay last week, and ordered another liter jar and 4 more bags after I saw how much I used. The 500ml/2 bags are in my 125, and I have 12 other tanks running. I figure 6 bags should be enough to occasionally swap around among tanks. Expensive stuff even in larger volumes, and the seachem bags are way overpriced, but still cheaper than buying carbon in the long run.