I've actually figured this out, so I think I can help. A 4.5g bag of granite costs me $6. A 10.5g bag of Turface costs me $12. And a 15g bag of fir bark costs me $28. My recipe has changed a bit since I no longer use bark, but if I used the standard 1:1:1 recipe, I estimate it would cost me about $1.75 per gallon including sifting losses. Prices will vary from location to location, but most people should be somewhere in that neighborhood if they're buying from the right sources (landscaping/feed stores). The eBay soil is $4.66/g before shipping, and $9.75/g after. That's a pretty substantial markup, and the shipping really puts it over the top. If you just have a few plants to repot and want to save yourself the labor, it may be worthwhile. But if you're going to be using a lot of soil, it's definitely worthwhile to make it yourself.

-Chris

napa floor dry 25lb approx. $10
manna chicken grit 50lb approx. $10
yard of pine bark mulch approx. $5

I have no idea about total yield I have just sifted the last of the original mulch I bought for this, ive had to buy additional de and granite, but I also use the de by itself semi hydro for some tropicals.

would love a better bark option in western ny/nw pa

I feel like I am misremembering those weights and I just threw the bags out, so you are welcome, for nothing :p

I just bought more supplies this past week for some overdue repotting.

Poultry grit (Manna) 25 lbs $8.00 I got this from Tractor Supply.
Napa 8822 24 qts (not listed in lbs) $9.00
Zoo Med ReptiBark (pet stores) 8 qts $8.00

The ReptiBark is actually chunkier than I'd like, but it's the best I can find. I smashed it to smaller pieces for some of my plants, but it has worked well for me out of the bag, too. The fir bark works best for me.

I mixed up a batch in a dish pan that is 12x14x6, about 4-5 cups of each ingredient. I filled six 4-inch pots, one 6-inch and one 8-inch orchid pot. I had a little left over - just to give you an idea of how far that batch went.

I have plenty of supplies left to make more batches of gritty mix for about $25.
Hope that helps!

Re: diatomaceous earth (DE)--

'A few items to remember when using Diatomaceous Earth:

-remember that you should only use Food-Grade Diatomaceous Earth. It is the only type of Diatomaceous Earth that is safe for home use. Do not use the type of Diatomaceous Earth that is used in swimming pool filters.

-Diatomaceous Earth is a drying agent and just as it will dry out insects, it will dry out your skin. It is useful to wear gloves and apply hand lotion after handling Diatomaceous Earth. Also be advised that Diatomaceous Earth will irritate the eyes. Diatomaceous earth is fine and will float in the air for a while before settling. Using a dust mask while the dust particles are still in the air will prevent any irritation caused by breathing it in.'

Here is a link that might be useful: about DE

there is some powder involved with the napa and agree you should probably take measures for the respiration and wash well. they are using now to treat for bed bugs. I have also used the food grade for digestion cleanse.

Very good point about wearing the dusk mask. The first time I sifted DE I did not wear a mask. I coughed for several days and had a terrible sore throat.