Several things here...
1. Have you considered other beach destinations? Santa Cruz and Monterey are much closer to the Bay Area than Pismo Beach, which is on the Central Coast.
2. Beach-front lodging probably won't be had for $100 unless you do not want a view or a particularly updated room. Considering your length of stay, you may want to try a vacation rental. Also consider using a site like Hotwire or Priceline if you would like to save money.
3. With a full week to spare, you will want to rent a car. Believe me I am an advocate for California's crumbling public transit system, but especially in a place like Pismo Beach, you'll get cabin fever without a car. (I lived in San Luis Obispo for four years without one, and I would not recommend the experience to anyone. In any event, this is one of the few cases where I can say the public transit system has *improved* over time. It's still very anemic, though. Public transit elsewhere has been on a regular decline for the past X years.)
4. To get from Berkeley to Pismo Beach, you will want to take Amtrak. There are three different route combos, so you'll have to decide which option sounds best. They are:
(1) Take the Coast Starlight train from Emeryville to San Luis Obispo. You will have to catch AC Transit Route 19 to get from Berkeley to the Emeryville train station, and SLORTA Route 10 to get from SLO's train station to Pismo Beach.
(2) Take the "Coast Daylight" bus from Oakland to San Luis Obispo, then either catch a connecting Amtrak train to Grover Beach or SLORTA Route 10. AC Transit Routes 72 and 72M stop at the Oakland train station near Jack London Square.
(3) Take the Capitol Corridor train from Berkeley to San Jose, then transfer to a connection Amtrak bus to Grover Beach. AC Transit Routes 19 and 51 stop at the Berkeley train station, and SLORTA SCAT Routes 21/24 operate between Grover Beach and Pismo Beach.