Here is my go to GF all purpose flour mix:
5 1/4 cups (25 oz) fine white rice flour
1 cup (7 oz) potato flour
1 cup (3 1/2 oz) tapioca flour (also called tapioca starch)
I use a food scale and weight out my flours rather than measure out in a cup.
I found this recipe in Phil Vickery's Gluten-Free Baking, he calls it Basic Flour Mix A. In that book Vickery limits the uses of this mix to cakes and cookies (I think) but I honestly love it for most of my applications rather than spending tons of money buying a pre-made all purpose mix. There were some other good recipes in this book so search ISBN: 9781554078110 for your own copy. I borrowed it from my local library and like I said I love this flour mix.
If the recipe calls for an all purpose mix with xantham gum, you can add 1/4 tsp of xantham gum per cup of this flour mix.
I borrowed this book from the library and Phil Vickery has no clue who I am so I have not been compensated for the post at all. I love this flour mix and why spend tons of money on an expensive mix when this one works so well?
I spent $6 on two bags of white rice flour (24 oz each), $4 for one 24 oz potato flour, and $4 for one 24 oz tapioca flour. The potato flour and tapioca flour will both stretch into another batch and you'll only need one bag of rice flour the next time so I think this is pretty thrifty since you get about 7 cups of mix per batch.