If you are looking to buy it in bulk make sure you are looking at these compressed bales
http://www.premierhort.com/eProMix/Horticulture/fHorticulture.htm
they are $26 at Rona, $28 at local hydro store. They are massive and it is compressed so theres alot more than you think, 3.8 cu ft (about 80 pounds DRY). I'm sure if you mention that you need 20 of them, they would give you a discount (esp at local hydro store).
Id recommend the HP (High Porosity) mix as it has a root stimulator that I find works wonders, and it has a perfect amount of coarse perlite with a wetting agent that makes it very hard to over/under water. Ive been using it for my outdoor crop and now my first indoor as well, with no complaints.
Although it contains enough macro & micro nutrients for 1 week, it is considered a soil less mix and they recommend you start a nutrient program within 1 week of planting. For seedlings it has enough to last 2-3 weeks easily Ive found, but for mature plants definitely start nutrients within 1 week.
Hope that was helpful.