Citizen Space (
http://citizenspace.us) might be a good option for you. For a small donation (they recommend $10 to $20) you'll pretty much have what you need for working on your app. You should give 'em a call just to make sure there's space available, however. Not sure how quickly the drop-in slots get filled up.
Of course, Starbucks now provides free Wi-Fi so that may turn out to be good enough. I occasionally use the 'bucks across from the Metreon (4th & Mission). It's right next to a parking garage (not all that cheap, however) but does have lots of power outlets, and there's plenty of decent food options within walking distance.