$5 for either a set of Ernie ball Slinkies or a set of DR's.
I buy as many sets as i can of discontinued strings for $2 a pack for weird things i make and want to test out- then, after they're ready to go, i put a $5 set on them.
I'm not one to change strings after every gig, so it works out just fine.
I recently bought some GHS Boomers at my local shop for $3.86/pack. The E Balls were $4.29 as were D'Addarios. I decided to try Gibson's Brite Wires on my LP for $5.99 (imho, they ain't worth it, I'll go back to the Vintage Nickel Gibby strings..but they are $6.99 a set now !).
How do you get em to do that? Bring in a print out of a cheaper price on strings and they'll match it?? Sounds like a good idea if so, the shipping on strings can be kinda ridiculous unless you get a lot of sets.