(Note: You'll need to acquire and use a Clipper Card to use 8-ride tickets, see https://www.clippercard.com/ClipperWeb/index.do - or just go to a Walgreens when you get here and you can buy the card and the 8-ride ticket at the same time.)
The 8-ride ticket was recently nerfed+. The discount is smaller, and you have to use all 8 rides within 30 days now instead of 60-90-whateveritwas. So if you're going up and back at least once a week, it's a small savings, but if you miss even one ride then you've probably wiped out several 8-rides' worth of savings.
I think they're doing a paper-ticket surcharge anyway so get the Clipper card, and consider paying with Clipper-loaded cash value instead.
(+ They were planning to get rid of 8-rides entirely and making it paper-ticket or 25c-discount-for-Clipper in something that amounted to a stealth fare hike - of course they denied that was the main motive).
From San Jose, you'd need a 4-zone ticket, which would cost $64.75 -- this gives you 4 round trips to SF.
http://www.caltrain.com/Fares/farechart.html
(Note: You'll need to acquire and use a Clipper Card to use 8-ride tickets, see https://www.clippercard.com/ClipperWeb/index.do - or just go to a Walgreens when you get here and you can buy the card and the 8-ride ticket at the same time.)