Here's what I'm playing with. Mind you this is for spots I don't want anything to grow. Areas include the driveway and along the drive. Most of the web suggestions mention vinegar (one gallon), salt (one cup), and some type of detergent (one tblspn) in varying quantities. I decided to go with some copper sulfate (bought at a local farm supply store) I use for the septic system. It's a salt and is supposed to sterilize soil.
Before...
After...two days later
Pretty good results the way I look at it. None will be applied in the center of the driveway. Question is how long will it last? I may add another shot with a copper sulfate and water only mix to get at the roots. The first application was sprayed lightly all the way up the driveway or 150 feet.
I would not suggest it to spot kill weeds in flower beds or lawns.
Costs?
Vinegar $ 2.65
Detergent ? A nickel?
Copper Sulfate? I'd guess $ 2.00 if you were to buy a two pound container of RootKill
Coverage in this example. Approximately 250 square feet.
Update June 19. Save your money. It doesn't work, but the application of copper sulfate and water was very weak.