Hmm, having been trying to duplicate what you're looking for on my listing. Have you tried setting your minimum and maximum days required to 30 minimum and whatever you decide is your maximum? Then setting you daily price which x 30 would equal the monthly price you're looking for?
Also, since you are in California, have you read this Airbnb article?
www.airbnb.com/help/article/805/what-are-some-things-i-should-consider-before-hosting-long-term-gues...
Unless you are experienced in long term rents you might heed their cautions since stays over 30 days can create a tenancy by your guest and if they don't want to leave, you've got some legal problems on your hands.
Hope this helps in some small way. Good luck!