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.