Soil mixes 101
I have been making my own soil mixes when I found that almost all ready made mixes contain dolomite lime. This makes the soil more alkaline but my water has a high ph so adding more could make plants struggling to grow. It is always a good idea to check with your local water source and see what the ph is in your area's water before adding any lime.
I use a basic mix of 3 parts sphagnum peat moss, 2 parts vermiculite, and one part perlite. The brand of Sunshine Peat Moss is getting harder to find because there are newer suppliers and the quality is not as good as the Sunshine brand. It is not as fine and has twigs in it. I have found the Sunshine brand at a few garden centers and many times in a hardware store.
I like the coarse vermiculite. Again it is difficult to find. Most stores carry the horticulture vermiculite and it is very fine. I found it held more water and kept the soil too soggy. You should try and see what works for you.
Perlite does not seem to have many grades of sizes so it is easy to find. Once I get all the ingredients I put them in an ice cream bucket and water them well so the peat and vermiculite soak up the water. It takes about a day for it to get saturated.
The most important thing after I get it all soaked up is to mix it well getting every bit of the peat mixed in. The peat can end up in the bottom of the container with the perlite floating on top. This is the most important step to do. If you don't mix it thoroughly you will have a very loose mix on top, and a soggy mix on the bottom of the bucket.
I have never bothered to figure out the cost of making my own mixes, but I do know that what is in my mix. Each grower is different, as we have different living areas and different size plants we like to grow but this is a basic start of making your own mix. If you find you think the mix is not working well, then try different size vermiculite. Try not to add too many variables at one time because you may not know what worked well or not well.
The November/December 2009 issue of the AVSA African violet magazine has an article "Everyone's mix is different". It is a list from well-known growers and as you will see they are all different, yet the same using peat moss, vermiculite and perlite.