Have you looked at the trolleys? San Diego has a really good transportation system. An all day pass is $5 a person (and you can avoid parking fees and gas). There is a stop right across from the pier (we saw it when we were there last summer for DS's birthday - we took the train down and the station is right there).
http://www.sdmts.com/fares.asp#daypass
DH and I are going to do this and head to Old Town San Diego (and probably some other places).
I just checked and it is easier to take the bus. If you go on the website and select the trip planner, it will tell you what public transportation to use. I tried it and it said it was 11 minutes by bus.
FROM: B ST PIER
TO: BALBOA PARK
1. walk 0.6 mile E from B ST PIER to Broadway at Front St
2. At 10:49 PM take the MTS BUS route 7 La Mesa via University Ave
3. Get off the stop on Park Bl at Village Pl at approximately 11:00 PM.
4. walk 0.1 mile N to BALBOA PARK
Schedules and maps for your routes
Bus route 7
Fare Information
Refer to the fare chart at top of the page.
Total Trip Time = 0 hr. 11 min.
That would make it $40 total for 8 people.