A friend tweeted about how expensive the hotels near Moscone Center in San Francisco are (for Macworld). I've tried more reasonably-priced hotels near Union Square (which isn't too far a walk), but none of them are worthy of a recommendation.
In most cities, I'd set the bar of affordability lower, but this is San Francisco, so let's say under $200/night. (And ideally the hotel has free, reliable WiFi or in-room internet.)
Check out Hotel Fusion just off union square; it's more of a 'European-style' budget/boutique hotel, I stayed there the weekend before xmas last year and thought it was fine, was < $100/night then for a single room (actual single, with a real single bed!). WiFi was free, small continental breakfast included too. Recommend checking out the reviews on Tripadvisor before booking; if you're expecting something Hilton-esque with large rooms, it's likely not for you, but if you want something that falls between hostel and Hilton, it may fit the bill.
Macworld 2012's own site has a page on hotel accommodation with quite a few hotels mentioned in the price range you want (~$200/night). If you click through to their partner site for hotel bookings, you can also sort through a list of hotels closest to Macworld venue sorted by distance (search for rooms on particular nights, then use the 'distance' sort on results page).
