Cheap, effective degreaser recommendations? Maybe it's just me, but most of the commercial degreasers sold in bike shops seem pretty expensive for what they do.

While they work well for lightweight grime, especially when used regularly, they just don't cut it when it comes to cleaning off months' worth of caked-on road grease and filth. Lots of time and elbow grease required (or a stiff brush, which can damage the finish on some parts).

Community mechanic that I am, I often end up sorting out friends' bikes for them. Now when it comes to salvaging a neglected bike, there is only one thing that is cheap and potent enough to make my task bearable: petrol (gasp).

Yes, I know. Awful for your skin and lungs, and the surrounding environment, and a stupendously good fire hazard, but it takes off just about any gunk in seconds. I realise that this may just spell death for a nice laquer finish, but I always use it quickly and sparingly, and I've never seen any ill effects to date. A nice rub down with oil and wax afterwards, and the job's a good 'un.

Seriously - 5 litres of petrol is cheaper than 500ml of degreaser. It is also 5,000,000,000 times worse for myself and everybody around me. Without hating on me for being a terrible being, can anybody recommend me a suitably cheap alternative?
Go to an auto parts store. In the case of cleaners and the like they typically sell the same things in larger packs and larger quantities than bike stores. They often also have a house brand that's cheaper than the other stuff.

You can get degreaser in the spray can (something like GunkFoam or also a non-foam version). Or you can get a solvent as used as parts cleaner. The linked one is 5 gallon but you can find it in 1 gallon as well.  This is good for soaking parts or dipping a small brush to clean parts.

You can also try household degreaser like Purple Power – it works better than you would guess.

And another trick is tire cleaner. For that profession detail look.