One factor in the price differences is that most chemicals come in different grades, which reflect different levels of purity. I don't recall what the various grades are, but you definitely pay a premium if you buy the stuff that's intended for scientific laboratories, and in some cases stuff intended for use in foods. Suppliers like Artcraft and Photo Formulary carry grades that are adequate for use in photochemistry. You can sometimes save by buying from other suppliers, but you might not get something useful. For instance, you shouldn't use iodized table salt from a supermarket as a source for sodium chloride -- as I understand it, the iodine will almost certainly damage your emulsion, and other ingredients in supermarket salt will at best throw off your weights.
As to sources, here are a few I've used:
- Artcraft -- You seem to have already found them. They carry most of what you'd need for photochemistry, and in appropriate grades.
- B&H -- This general photographic mail-order house carries some of the more popular items, mostly sourced from Photographer's Formulary (see below).
- Digital Truth -- Their selection is slightly less than Artcraft's or Photographer's Formulary, but they've got good prices on many things. They cater to photographers.
- Photographer's Formulary -- This is probably the top US photochemical supplier for hobbyists. They've got the widest selection (although the last I checked, Artcraft had a small number of items PF lacked), they cater to photographers, and they sell kits of certain popular formulas. They tend to be pricier than many alternatives, though.
- Summer Bee Meadow -- This outfit sells soap-making supplies. They happen to have excellent prices on sodium hydroxide and potassium hydroxide, and will ship these items in small quantities (shipping these items can be tricky because they're hazmat items). IIRC, they've also got protective goggles and gloves.
- The Chemistry Store -- This company caters to soap makers and miscellaneous home chemistry uses. Although they lack many photochemistry-specific items, such as metol and phenidone, they've got a lot of things that you'll need in large quantities, such as sodium sulfite and sodium thiosulfate. Their prices are very good, but their shipping is a bit more than some others charge.
I recommend you make up a list and check for prices and availability of all items. You might find that you won't be able to get everything from one source, or if you can, it'll be the source with the highest price on everything else. (This is probably a corollary of Murphy's Law.) Thus, you might have to buy from two or three sources.
