Homebrew degreaser

I used to use a vapor degreaser to clean parts back when I was running a decent size shop (not bike shop). It was economical and eddective using halogenated solvents, but when the restrictions on CFCs went in to effective, the cost of solvents got too high, and I phased out it's use.

For home use on bike parts, a coffee can and solvent such as mineral spirits or naphtha work very effectively with a few rinses. The solvent can be recovered and stored in a closed container. 

"Generally once you've dissolved the grease, you then need to move the grease solution off the surface you want to degrease. Typically this would involve flushing with more solvent."

In hot water mix Dawn dish detergent. Use stiff brush but not so stiff it mars paint surfaces. Rinse with warm clean water.

You could try the homebrew beer my wife makes.

Not homebrew, but environmentally friendly (being citrus-based) and bloody effective - Virosol.

If you want to recycle something that would otherwise go to waste - do you own a car and service it yourself?  I've used old engine oil to degrease car engines - can't see why it wouldn't work on bike bits.

Educational read!

The internet will tell you that you can clean anything with a combination of lemon juice, white vinegar and bicarbonate of soda. This is mostly crap, and many home made cleaners perform no better than water. No bad thing as water is pretty good solvent for most household cleaning tasks. You just feel better as there is a smell. You need to pick the right solvent for the right solute. When trying to dissolve grease your talking about aliphatic organic liquids. e.g. Diesel. So to answer your question, one every day ingredient that can be used is diesel. If you want a 'green' choice, go for a plant derived organic solvent rather than petrochemical such as proper turpentine (most turps is now derived from petrochemicals). Generally once you've dissolved the grease, you then need to move the grease solution off the surface you want to degrease. Typically this would involve flushing with more solvent. Otherwise the solvent evaporates leaving behind the non-volatile grease. Many commercial degreasers will contain some form of alcohol as a sort of half-way between aqueous and organic solvent. This will allow you rinse off the degreaser with water. Look at the material safety data sheet (MSDS) for a commercial bike degreaser they're freely available online and it will breakdown the ingredients for you. Most are a mixture of organic solvent (think diesel). Small part of alcohol to make it washable with water and some fragrance, typically limonene so they can call it citrus degreaser and make you think it's environmentally sustainable. 

StraelGuy's suggestion of homemade lye would probably work. High pH solutions will indeed chemically change oils into soaps through a process called saponification. This is how oven cleaners and sink unblockers work. Would still require a bit of scrubbing as they're not hugely soluble. If you wanted to try it out try cleaning your chain with Mr Muscle. As StraelGuy suggests making your own caustic from wood ash would likely lead to the deposition of particulates all over the bits of your bike that are normally lubricated causing more issues than it solves.

I applaud your efforts to stray away from commercial products as a lot of them you're just paying for the marketing wank. Why does washing up liquid foam for example, no one is washing their dishes with the foamy bit. However in this instance there is no environmentally benign degreaser available commercially let alone homemade. If you want to save the money just use diesel. 

The only thing I can think of is ash from a fireplace. If you mixed it into a slurry with some water, it would be mildly caustic and would probably make a decent hash of degreasing a chain. Messy though...