Re: WD-40 on strings
WD-40 is purely a lubricant and it dries leaving no rust-preventing coating (learned that from a police armorer who hated seeing officers clean their gun with it)
I wouldn't get it anywhere NEAR my guitar finishes, cheap or not.
Spraying it on would also ensure that it would pool around the frets, soaking down into the fret slots, damaging the wood that holds your frets in!!
If we were living in 1929 during the depression, I could understand but geez, shell out a couple of dollars for real string cleaner.