FWIW, I put together a spreadsheet recently with various photochemistry
formulae and costs. I put much more effort into this than was really
justified in terms of the potential for identifying cost savings, but I
was curious about it. Anyhow, I got costs of $0.51/tray for
working-strength (1:3) Dektol vs. $0.32/tray for working-strength
home-made D-72. (Each tray is 750ml.) These prices all include
shipping/tax charges for chemistry, which can be a substantial fraction of
the total cost. For the most part, I assumed buying enough chemistry for
about 10-50 batches of a typical formula. If anybody cares, I'm willing to
share my spreadsheet; e-mail me if you're interested. (It's in
OpenOffice.org Calc format, but I can export it as Excel or in various
other formats.)

As a general rule, the best prices I've found on bulk chemicals are at The
Chemistry Store (http://www.chemistrystore.com), but their shipping rates
are high. They're also not primarily a photochemical supplier, so you
can't get stuff like metol or phenidone from them. For that sort of thing,
Digital Truth (http://www.digitaltruth.com/store/rawchemicals.html) or Art
Craft (http://www.artcraftchemicals.com) seem to be the cheapest.
Photographer's Formulary (http://www.photoformulary.com) is an oft-cited
supplier, but aside from a few items others don't carry, they seem to be
pricier than the others. A few items (sodium carbonate, borax, etc.) can
be found in supermarkets or drug stores for far less than the mail-order
suppliers charge. I'd certainly be interested in hearing of other
suppliers, although I personally just went on a chemistry shopping spree
and doubt if I'll be needing anything else for a while. :-)