<<Disclaimer:  Verify this information before applying it to your situation.>>

> Ener-G sells it in five pound bags.  It's probably a little more
> expensive than mixing (I've not figured it out) but to some, well
> worth the convenience.

In looking at the Ener-G order form, I see that the GF mix is $2.39/lb.
If you buy the three flours separately from Ener-G and mix them yourself,
it comes to $2.43/lb., which is actually slightly more.  However, we buy
our white rice flour and tapioca starch at a local India Foods store for
substantially less money ($0.40/lb. and $1.09/lb., respectively).
Therefore, it only costs us about $1.14/lb. to make up our own GF mix.

Caution:  Many smaller specialty stores package the flours themselves.
Be sure to talk to them and ascertain that they clean the scoops
thoroughly after each use, before you buy locally-packaged products.