Personally, when it comes to choosing the best substrate for my wild bettas, I want a substrate that is either inert, or has a softening effect on my water. It should have added nutrients to promote plant growth, and come in a range of natural colours, preferably in medium or dark tones. As I only use a thin layer of substrate in each tank, and a 9L bag of substrate can last me for some time, cost is rarely something I consider, although it obviously can't be too exorbitantly priced.
In 2015, my two favourite substrates for my wild bettas remain ADA aqua soil and peat moss. I like ADA aquasoil because it looks very natural, comes in a range of colours, doesn't increase the pH or water hardness, and provides nutrients to my plants. The only downside is probably its price as it costs around $40-50 (plus shipping) for a 9L bag.
With that said, there are dozens of brands of aqua soil available to hobbyists, so I assume that there are cheaper options out there. I personally go with ADA because it's a trusted brand, has a proven track record when it comes to plant growth, and I like the range of colours it comes in.
One downside to aqua soils (particularly ADA) is that some will release large amount of ammonia. I think ADA Amazonia is one of the worst and I recorded 8ppm of ammonia when I used it in one of my tanks. Even though at a low pH ammonia becomes much less toxic, I still feel it is not healthy for fish to be exposed to it for extended periods of time.
Peat moss was what I used before aqua soil. I would simply boil up some peat, release several handfuls into the water, and then give it time to settle on the bottom of the tank. The benefits of peat moss for me, are its dark colour, and its acidifying effect on water. Unfortunately, it can also make a mess when disturbed (the main reason I don't use it as much now), and there are environmental concerns in regards to its harvest, which I have touched on before in this blog.
However, in spite of its drawbacks, I still feel peat moss has its place in my fish room. Nowadays, I use it more commonly as a substrate in grow-outs to promote infusoria, and in tanks housing individual adults where I'm not going to be netting or removing fish on a frequent basis.
As always, this is only my personal opinion. I have seen hobbyists using a wide range of substrates in their wild betta tanks and still having great looking fish and breeding success. It really all comes down to what you and your fish are most comfortable with.
Hailing from Melbourne, Australia, I currently keep and breed a number of species from the coccina complex.