Yeah, heh. 25gp worth of powdered silver.
It's not so much the holyness, or really the water that does the damage.. it's the fact that you're clocking the evil with a couple pounds of moist silver dust. Sort of like a Sacred Sap, really. To imagine what it's like, just take your average sock and fill it about 2/3rds of the way with sand. Pour in a tablespoon or two of tap water, knot it closed, and viola - "Holy Water"