Would depend on the contents of the "shock". If it was dichlor then as far as chlorine goes its pretty close to the same dose as a gallon of 6% bleach. (Though bleach normally comes in 96oz containers) If it was cal-hypo then it would depend on the percent concentration on the packaging. However both dichlor and cal-hypo have effects on other areas of pool chemistry. Dichlor will raise your cya and cal-hypo your calcium hardness. If you need a boost in these areas then it's probably not such a bad thing to use them occasionally but not as your primary source of chlorine, even if you do find some super cheap.