Generic non-chlorinated brake cleaner is usually a mixture of acetone, toluene, and methanol. Looking at the MSDS for a few different types seems to confirm that these 3 ingredients are pretty common to most of them. The MSDS also hints at ranges of proportions but doesn't give exact ratios for proprietary reasons.
I have mixed my own in the past from ingredients purchased at the local hardware store. Each of the three listed chemicals costs around $12-15 (USD) in gallon quantities. Aerosol cans of brake cleaner are usually around $2-3 (USD) per can, but much of that is propellant, and though I haven't done the math, I suspect that purchasing in quantity and mixing my own saves me a bit of coin.
I mix in proportions of 50% acetone, 25% toluene (or xylene if toluene is not available), and 25% denatured alcohol (AKA methylated spirits, AKA ethanol with methanol added to make it undrinkable). If you want true methanol, it is often sold for a few dollars a gallon at places that cater to race cars and at some hobby shops. I've found methylated spirits to work fine, but I've also never washed circuit boards with my witches' brew. You can also find true toluene at some hardware stores (at least here in the States), but xylene seems to be more widely available in my area.
Acetone can be very aggressive on some plastics, as can xylene or toluene. ABS, polycarbonates, PVC, and a few others will dissolve in acetone, xylene, and/or toluene.