Rent: I live on Nob Hill in SF. I pay $1950 for a big, nice one bedroom with a little office and a small private yard. (100sq feet.) When I signed the lease I was making good money so it didn't seem expensive. Since then I've decided to focus on my start-up full time, so now it does seem slightly pricey. (I only pay 50% though -- I'm coupled.) I've lived in New York and Seattle, and SF is nowhere near as expensive as New York and nowhere near as affordable as Seattle (in terms of renting, not buying).
Beyond rent, things are very affordable.
Transportation: cheap, cars are optional (I don't need one; I hate mechanics and oil changes and parking...). I get my groceries delivered and use ZipCar when I have to drive. This is a huge savings and a good reason to live in the heart of the city. Instead of a gym, I just walk up the hill everyday.
Healthcare: I'm on Cobra right now, but will qualify for SF City HealthCare for free soon, as I'll be making little money. This is a big reason to live in SF and not the peninsula. http://www.healthysanfrancisco.org
Entertainment: The most expensive thing I do is eat out with friends semi-regularly. However, I've found that for geeks, there are a ton of meet ups and other social events that are 100% free -- and they're usually the best way to have fun.
Other threads are noting the cost of rent in SF, but for me, the city health care coverage and not needing a car (or insurance, or gas, or parking) more than makes up for it.