bags of shock vs bleach

To raise the chlorine level to shock level in my 20k gallon pool I figure it would need 8 jugs of bleach at 3 bucks each, that's $24.00. I was looking on the online pool stores and they sell 1lb bags at 2.75 per bag and claim 1 bag per 10,000 gallons. so 2 bags for my 20k gallon pool at 2.75 per bag that's 5.50. I'm new at this so please let me know if I am being misinformed. Please educate me. I completely understand the bbb method and it is working awesome. My water has never looked this good and it is so simple to do. I still have my chlorine feeder in place and thought it would be good to use when I go on my week vacation. I am currently having to pour in 1 jug a day . I'm thinking the 3" tabs would work while im gone. How long does a single 3" tab last in a feeder. I know it will raise my cyn level. This is a outdoor above ground pool. how long does it take for the the cynic acid to go away. I know at the beginning of the year my level is always zero.

A few questions:
-- Any idea what your current CYA reading is?
-- What kind of test kit are you using?
-- What are the ingredients of the bags of shock?

While the instructions on the bags of "shock" (btw, technically "shock" is a verb, not a noun ;) ) do say use 1 bag (1lb) per 10k gallons, 2 bags isn't going to give you near the CL level that 8 jugs of 8.25% bleach will give you in your 20k gal pool. You really need to know how much available CL is in each bag and how many ppm each will add to your pool to determine how many bags you will need to reach the same CL level as 8 jugs of bleach. Then you can make a valid cost comparison.

IMO, even if bleach ends up costing more, I'd still go that route since it doesn't add anything else that I don't want or need in my pool. :)