Is it all super expensive?
You can try sleeping on the streets...that seems to be a popular option with many people in San Francisco. I've never seen more homeless people in one place in my life!
Seriously, accommodation in SF is very expensive. I usually don't recommend AirBNB and instead recommend that people stay in reputable hotels, but since hotels are so overpriced in SF then AirBNB might not be a bad option. Or, you can stay somewhere else (e.g. Oakland or Berkeley) and use BART (the subway/rail network) to commute to the city. The Oakland Marriott City Center and Hotel Shattuck Plaza are very nice places that sometimes offer discounted rates, and each is within 2 blocks of the nearest BART station.
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If you're not opposed to (Youth) Hostels, Fort Mason Center is nice, reasonably priced, limited breakfast included, and well located. I believe they have some family rooms available.
ALL expensive but over the years(visit almost yrly for 27yrs.) have found good deals by searching the internet & going on individual sites for hotels.