That is lower than most non-technical positions for medium to large companies. For startups I can understand if it comes with a chunk of equity, but for $70k, you will have a hard time meeting living costs.
What platform would you be writing on? Unless you're doing HTML cutups or working almost exclusively in WordPress, 70k is a joke.
The company's software stack includes php and python. But on average, what number should I give? And start-ups vs larger companies?
In SF you should start at no lower than 90k. I would start at 110k and let them work you down to something more reasonable with concessions.