It is fairly inexpensive. I got 2 bags of it from the local micheals for $4.99 each. The stuff I picked up is in a plastic bag that looks like a small pillow. It is a little packed in and is just the loose fibers called poly-fil for stuffing bears and pillows and such.
The loose fiber you can adjust if you want (fluffing it out to hold less water/have more air room) The batting is more like what you would see as a cusion topping in your sofa. you can get either. It is a matter of shopping around to find where you can get it inexpensively. Some places want to charge a lot for a little..... Remember this is the same stuff that is used to make soda bottles, and is not expensive. (the same things used to make those little plastic cups you use most likely as well). The same plastics have a million different names depending on how it is packaged for the consumer.
The bag is about the same size as a bag of perlite only much fluffier. It does act differently than perlite and such so you might want to watch how it reacts in your system at first, but I am really enjoying using it. The two little bags of fluff will probably last me a while. Atleast till I fill up the giant net pots for my tomato plants :P
But it is reusable if you can get it cleaned out. I am going to see what some sulfuric acid does to it (if anything considering the battery boxes are made of similar plastic) It might be a good option for cleaning (dilute Sulf acid). You can get some decently strong acid from an auto store for less than $10 and it will make a lifetime supply of PH down lol... I might as well check it for other uses as well :P though I could always rot out the roots that get caught in after a grow and then sterilize after if I wanted to be a bit more "thrifty" about it. It all depends on how you want to use it. The material is VERY easy to work with once you notice it's tendancies with water holding and such. I leave my pump
spraying on my plants 24/7 and they don't complain about too much water. It seems to hold the water just how the plants like it.
PS: I just opened up a bag to see how many 6" net pots (the ones for 5gal buckets) it would fill... I filled up 5 of them and the way the stuff expands when I pulled it out of the bag, I can barely tell I made a dent in the bag after filling all 5 of the 6" pots.. The stuff is very soft and light. VERY comfotable to handle. Imagine your plants resting comfortably in the guts of a fluffy teddy bear :P The kids were all about helping and they were rubbing a piece on their faces after I told them to stop putting it in the pots :P
I think the tomato plants will enjoy it. The roots will have to resistance growing out wherevery they want. And if the wieght to the plant pushes the batting down it will be easy to just add a little more to the top. (the amount needed to fill the 6" pots will squish down to nothing with a little wieght.)