i've found that (believe it or not), drs. foster and smith has the best prices for eco-complete--it's the shipping that kills you, but their bags come out to about $24 incl. shipping (at least to ny). that's where i bought mine. i also gthought of doing 2/3 eco and 1/3 tahitian moon sand. i opted for eco-complete and regular estes "natural" gravel (in dark and light--see my gallery), because i like the look of the natural gravel better than a straight black substrate--purely aesthetic. i've also mixed eco and flourite, again purely for aesthetics--to break up the dull black--and thought it looked nice too.
with my current setup (gravel over eco) the only thing i've noticed is that i definitely have to use root tabs for the heavy root feeders like my swords. otheriwse, it was a little more cost effective, and aesthetically satisfying.
hth