There's a gallery in SOMA called "One Taste" that donated the gallery for a fundraiser taht I was doing...I think we paid a nominal fee like $200...they have a kitchen that we used to set up our own hors d'ouvres (you can stock up on cheap sushi and etc at Trader Joes and arrange it artfully to look as if you have had the event catered while only spending $150). However, this space is pretty small. You could squish 100 people in, all standing.