good place for party of 20-30 in san diego for affordable wedding rehearsal dinner??
I think Sammy's woodfired pizza would be a great spot. they can accomodate large groups at most of them, are quite affordable, and yet quite "upscale/classy". I particularly like Del Mar and think you can reserve the back area (would seat 20-30 or maybe more). Food is great too. As well there are a number of other locations (Mission Valley, Point loma, La Jolla, la Mesa etc).
How much are looking to spend per person?
Roys in La Jolla Village Square has a private room and they can arrange a great menu.
On the cheaper side, you could ask to reserve the patio at Alphonso's in La Jolla and pre order platters of mexican food and their nachos and don't forget the Margs.