PBW is awesome. Unfortunetly, it's also expensive. Many homebrewers (myself included) turn to Oxiclean as a suitable alternative. It's main ingredients are pretty similar to PBW -- sodium carbonate (washing soda), and sodium percabonate (the oxidizing agent). The problem is: Oxiclean simply doesn't work as well as PBW. It doesn't remove caked on crap as well as PBW does, it doesn't rinse away as easily, and if you have hard water, it can leave hard water scale on your equipment. So where's the missing link?
PBW has a few other ingredients. First off, PBW contains a surfactant. Surfactants lower the surface tension of a liquid; they essentially make water wetter. This aids the detergent in removing soil from surfaces. Oxiclean also contains a surfactant (Ethoxylated Alcohol C12-C16). I don't know enough about chemistry to know whether it's as effective what's used in PBW, but it's there.
PBW also contains a chelating agent. Chelators bind with metal ions, which prevent them from reacting with other elements. This is why PBW doesn't cause scale on your equipment, even with hard water. Oxiclean unfortunately does not contain a chelating agent. EDTA is a commonly used industrial chelator. I'm not sure if that's what PBW uses, but EDTA isn't easy to buy, so we'll skip it for now.
Finally, PBW is more alkaline than Oxiclean, which is due to 30% of the formula being sodium metasilicate, a TSP alternative. Tri-Sodium Phosphates are no longer for sale in most states, as phosphates are really bad for the environment, but there are substitutes on the market. TSP/90 is a popular one made by Red Devil. It's active agent is sodium metasilicate, which is the same ingredient in PBW. Thankfully TSP/90 is cheap, and costs about the same per pound as oxiclean. You can find it at most hardware stores.
So now let's get down to the meat and bones. If you mix (by weight) 70% Oxiclean with 30% TSP/90 (sodium metasilicate), you end up with a pretty passable PBW alternative. The proper concentration to use is around 1oz (by weight) per gallon of water. That handy little oxiclean scoop has number on the sides, and each number corresponds with roughly one weighted ounce. So if you're mixing 3 gallons of water, fill the scoop to the '3'.
I've found the DIY PBW works flipping magic compared to oxiclean alone, and from my experience it's cleaning power seems to be about on par with PBW. Unfortunately, our homemade mixture doesn't have a chelator, so hard water scale is still an issue for those with hard water (like myself). I'd really like to speak with a chemist regarding using EDTA or EDDS to aid in chelation. The toxicity of both are pretty low, but I have no idea how they would react with what's already in the mixture. Dicking around with chemicals isn't something I take too lightly, so I'm going to hold off on that for now.
That's about it. My DIY PBW has worked pretty well. Give it a try, and let me know how it works for you.