I haven't done the cost per use breakdown so here it is: 96 oz of 6% bleach costs approx $1.50 and a 400 gallon tub takes about 2 1/2 oz to reach 3 PPM (I will use 3 oz for calculation simplicity), that would give a months worth of daily doses (actually 32 days) without using it for shock; 1.50 x 12 = $18.00. Dichlor would be around $30 so it is a little cheaper but you still need dichlor before using the bleach to get some CYA (20 to 30 PPM) into the tub.
Now for some thoughts on bleach vs all other chlorine. All chlorine acts the same on bacteria or algea as long as you use the proper amounts to get the desired effect. Trichlor adds CYA and is acidic, Dichlor adds CYA and is neutral, cal hypo adds calcium and I believe is alkaline and bleach adds salt and is alkaline. All chlorine apparently adds salt but 1/2 of what bleach does and all chlorine adds to the TDS of the water.
My water's PH stayed about the same while using bleach. Apparently, the reaction of using the disinfection ions (can never remember the names of the 2 ions) is acidic and counteracts the alkalinity of the bleach.
I can't comment on the "lasting" time of bleach in my tub because my tub seems to retain chlorine at times whether it's from bleach or dichlor; if anything I think it lasts about the same but I can't be positive.
The CYA is the real issue as it builds up over time. Yes, chlorine does kill most bacteria at a 300 PPM level but it's power is greatly reduced. If you use 3 PPM of dichlor at a time it adds about 2.7 PPM of CYA.. If you used the tub 6 days and shocked to 10 PPM on the seventh that would be 2.7 x 6 = 16.2 + 9 (shock) = 25.2 PPM per week; so 100 PPM can be achieved in a month depending on use. Of course not using the tub will reduce the amount of CYA in the tub. After about 30 PPM CYA chlorine has lost it's effect on the hot tub itch bacteria if it gets introduced. This bacteria needs a total contact time (minutes) and dose (chlorine) number of 2500 to consider it disinfected, chlorine effectiveness loses it's strength as CYA builds and higher levels of chlorine are needed to keep this bacteria in check.
The truth is - most people don't have to worry about the hot tub itch bacteria, using bleach seemed to be a little more of a hassel than granuals, draining water every 3 to 4 months is the most effective way to deal with the water, some people seem to want to keep their water forever and learning something new is FUN!