Miracle Grow Organic Choice Potting Mix. You can search for it on this forum. I would advise you to take the time to pick out or sift out all the larger particles like wood chips. I got lazy and didn't do it, and kind of regret it now. Hopefully my sand cap will be enough to hold the floaties down.
In terms of cost, I want to do it for cheap, but I still want quality, so I save when I could and spend when I should. The MGOCPM is a much better saving if you are doing a big tank (40+). I paid $10 for a huge bag and only used about 1/6 of it if that. Still cheaper than ADA Amazonia, but savings would've been exponential if I had a bigger tank.
Thanks for the response. I am working my way through my first planted tank. I am just using the Flora Substrate from the store because I figured it was easier, plus since I am keeping it at school and using it in my science classes, my school is going to pay the expenses for it. I am still trying to keep the cost down, but it is not as important right now.