Apple cider vinegar can be diluted half-and-half with water for external application, and, when stronger measures are needed, it can be used full-strength.
1 Just the stone for a flagstone patio can cost $3 -$5 a square foot or more, depending on location and the type and quality of stone, or about $600 -$1,000 for an average patio up to 10'x20,' and $1,200 -$2,000 for a larger 20'x20' area.
1-1/4 flagstone= approximately 16.7 pounds per square foot. 1-1/2 flagstone= approximately 20 pounds per square foot. 1-3/4 flagstone= approximately 23.33 pounds per square foot. 2 flagstone= approximately 27 pounds per square foot. If you are using it for a footpath, 1/2 will probably work. At your price of .13 per pound, 109.75 total.