I think there are a few reasons.
1) Rice flour on its own is not great to cook with. It's very dry and doesn't stick together. That's why there aren't too many recipes that just use rice flour alone. Generally, to make a recipe work well, you have to use an elaborate little combination of rice flour, tapioca flour, potato starch, and maybe some xanthan gum. Same thing with potato flour -- it doesn't work well by itself.
2) Rice flour also lacks nutrition, at least if it's white rice flour, which is basically empty calories. Brown rice flour might be a bit more nutritious. However, most celiacs already eat a lot of rice and rice pasta and do not need more of it in their diets. Nut flour provides very healthy fats and proteins that rice flour does not. Potato flour is also not terribly nutritious, and a good percentage of celiacs also need to avoid potatoes.
I do not think any of these recipes would work if you just substituted in rice flour for the existing flour, no. You would have to do a lot of experimentation and modification, which also gets expensive and results in a lot of wasted food. I would look for recipes which are already intended to be made with the flours you want to use.
Check out the Elana's chocolate cake with beets in it. That one is designed to be less expensive. She also comments on some of the things you are asking about in her post about that cake.
You might want to try ordering a 5 lb bag of almond flour from Honeyville. It's only about half as expensive as almond flour is in the natural food store. And keep in mind that the recipes made with almond flour are sooo much more filling and nutritious than recipes made with wheat or rice flour. For instance, I can eat half a pan of regular brownies easily. But almond butter brownies are so filling that I can only eat 1 or 2!
If you are on a serious budget, though, as I was a couple of years ago when I was in school, I recommend going to a Chinese or Asian grocery store and buying tons of rice noodles. They usually sell them very, very cheaply, and you can then make a ton of different pasta dishes out of them so you don't get bored. Get a rice steamer, too, and buy a huge bulk bag of brown rice. This will last you for months. Get creative with using beans and inexpensive veggies in your dishes. Make bean soups with whatever veggies are available to you. I have lived on lentil stew and rice for weeks at a time very happily, healthily, and cheaply! Make your own hummus. That's very cheap and healthy also. And try Elana's Purple Velvet Torte! http://www.elanaspantry.com/purple-velvet-torte/