I have at leaset 5 bags in my 46g bow, so I'd say 8 bags to get proper depth (6 minimum). That's why I decided to go with the charcoal color of Soil Master Select from Lesco (Pro's Choice). One 50lb bag takes care of a 75g tank easy (about $16!!!). You have to use way more Eco or Fluorite because they aren't as dry or light as Soilmaster. Heck your probably paying half the cost of Eco for the water in it...good for small tanks, but not cost efficient for large tanks.
I have 3 bags in my 75T and I think i could do for another 1-2 bags to get a little better substrate depth. 3 bags barely gets me 2" of depth.
Bioteach has a good suggestion of adding Soilmaster to the mix. its much cheaper and still has a similar color to the eco. they would mix well. Lesco.com and you can get a big bag for 16 bucks.+shipping
But if you can't find a Lesco store near you, then go for Shultz Aquatic Plant Soil.. Found at any store with a pond section.
Great stuff as well, but the charcoal colored Soilmaster looks a bit better.
You will save muchos dineros going this route.